This question is hard because nobody really has an answer. Going by the current theory of evolution I can come up with an idea. For millions of years the dinosaurs had been spreading across the earth. Both plant and animal life was plentiful. Then say there was a chance mutation among a certain meat eating dinosaur. It was just naturally bigger than all the others of his species. This allowed him to survive longer and pass on its genes more often. After a few million years, the smaller animals of this species had died off and now only the larger ones remained. During this time, other species of preditors were undergoing the same process as a direct result of these other, larger preditors. Also, smaller and weaker plant eaters died out as a direct result of these "super preditors" and only the larger ones remained. The process continued for millions of years until you have dinosaurs as tall as 3 story buildings. Then there was a huge catalysim and all the dinosaurs died out. An ice age proceeded and at this time was when modern mammals first appeared. I imagine that there were relatively few plants and animals around at this time. The environment could not support giant, 20 foot tall creatures. It was probably easier for smaller creatures to survive, as they would not have to consume so much food everyday. Now, say that mammals would undergo the same process that made the dinosaurs so huge. Not enough time has passed since modern mammals first appeared to allow for such huge growth. The dinosaurs existed for many more millions of years than modern mammals have. If humans never appeared and 50 million years had passed from now, maybe you would have dogs the size of elephants. Who knows.

Current theory, as I understand it, holds that life started in the oceans as single celled organisms. A fluidic medium is required for these single celled organisms to have any control over their intereaction with the environment around them. A fluid, specifically water, is dense enough to allow the self-propulsion of the single celled creature so that it can find and ingest food sources. Most gas media, such as air, are significantly less dense, and any means of locomotion would be capricious and dictated with the currents of the medium. The original multicelled creatures were little more than large colonies of single celled organisms. These colonies can take up vast amounts of space. As the varying portions of these colonies became more and more interdependent, multicelled organisms eventually took on the organizational patterns that we are familiar with in today's multicellular creatures. Water environments nullify much of the effects of gravity, allowing many of these organisms to retain their great size. Even today, the largest living creatures on the planet are whales. As creatures moved from water to land, they retained much of their great size. First these creatures had to adapt the proper physical structures to become land dwellers. Fins and flippers became legs and feet. Gills became lungs. Once these creatures had adapted to being able to support themselves on land for the long term, gravity would then have her say. Regardless of how successful some of these large creatures were, smaller forms are more efficient in higher gravity environments. The larger the creature, the more energy needed to maintain its ability to survive. For land-based creatures, there is nothing supporting them but their own bone, sinew, and muscle. At least a whale has water to help support its body weight. Not only that, large animals are easier targets for predators, no matter how ferocious they are. A single large animal is rarely a match for a pack of smaller ones. Smaller prey animals can hide much easier than larger prey aniamls. Also, Larger animals will deplete a given food source within an area more quickly than a smaller animal. Smaller animals usually breed more quickly, so they are able to take less food individually but take more food collectively. The size of creatures continued to decrease until they were more compatible with the environment they lived in, hence today's smaller size creatures relative to the large size of the dinosaurs.

But wait, there's more...DOSJockey was on the money, but there are other contributing factors as well. The largest of the dinosaurs, despite their structural problems, still dominated the planet for millions of years. It's not just structure that determines an organisms size limits but the availability of food energy to support a large organism. During the dinosaur age the earth experienced millions of years of stable temperatures and solar energy which encouraged massive flora growth. Essentially everything was a tropical jungle or temperate rain forest. Massive amounts of food that grew like weeds were available for plant-eaters. Now, a sidelight here is the example of fish and some reptiles today. They will grow just as big as their food supply and environment will allow up to their biological limits (i.e., the ability to regulate temperature, move blood, and so on) and so did the dinosaurs, except that a constant external temperature made internal temperature regulation unnecessary, so they never evolved the hot-blooded sytem of the mammals. With abundant food supply the plant eating dinosaurs grew enormous. When the enormous plant eaters died they became food for enormous scavengers, and both encouraged the formation of enormous meat-eaters, but all had the same vulnerability (cold-blood) that the tiny and inconsequential mammals had under control. A severe ecological shift occured that either caused, or resulted from, the extinction event that killed the dinosaurs. The average temperature of the planet dropped, sun-energy was blotted out, flora died and the plant-eaters along with it, and so on up the food chain. Only the mammals were equipped to survive. They were unable to grow very large because food was most likely scarce for many centuries. Furthermore, by the time resources became plentiful there were likely millions of little critters all over the place competing for the same food so that the survivors had to be able to survive on small amounts of food, thus a small organism. Whales still exist because the average sea temperatures at the depths they live were unaffected, and some mammals have grown quite large, only to die out when their environment was no longer capable of supporting them (i.e. Wooly Mammoth, Giant Ground Sloth, etc.) Therefore it seems that smaller is the more efficient, more numerous and more successful way to go.

Okay, here we go... Life started small, and simple. It became more and more complex over time. Indeed, life probably started as a single _molecule_, not single cell. Cells came (relatively) much later. I take the nugget of the question to be, "why were dinosaurs bigger than animals now?" Bottom line is, because their environment caused them to be. That is the answer to any question about any organism's features, more or less. There are oddities, mutations, and other things, and these are needed, to ensure evolution _continues_. (And yes, it is going just as fast now as ever!) But, why do birds fly, why do fish swim, why are whales big, why are mice small... all because of demands of the environment. This includes the dynamics of the environment, which, incidentally, is what squeezed the dino's out of existance. Interestingly, we jump to conclude that bacteria are "primitive". In fact, bacteria are _more_ evolved than us! They have gone through more generations, with more total trials (individual lives) since their ancestors came into being than we have (and we share ancestors!), and are arguably better suited to their environments than we. True, the organisms we find in hot pools and in places very close to what we believe were the conditions when life arose are similar to the early organisms, _because_they_never_moved_out! They are in essentially the same environment that they've been in for billions of years, and haven't had the selective pressure to change much. But they are really good at living where they live! (Actually, some lineages of organisms have moved out of and into different environments repeatedly [sea mammals!] and it is possible that the thermophiles we see today may have ancestors that lived elsewhere.) Don't listen to blathering about something in which you have an interest. You have the internet, search for stuff, go to trusted content providers like http://www.loc.gov, http://www.britannica.com, and anywhere else your curiosity may lead you. Be cautious, though. The old addage about not believing everything you read is especially pertinent on the internet, and when you seek truth, you may first find opinion in disguise.

The deeper into the ocean you go, the bigger animals tend to get. Giant Squid naturally live deep in the ocean, miles beneath the surface, as do some sharks. Whales are also big, for some dive miles into the ocean to hunt their prey. And why? Because the amount of presure deep in the ocean is worse than Earth's gravity, and being so huge counter acts this. Now since most of what we "know" about the dinosaurs comes from what we know about our animals of today, it is logical to think that this applies to waters creatures of long ago. Now, for whatever reason, these creatures moved from the deep waters to more shallow water, and then moved to land. This didn't take so long as to where these creatures would evolve to smaller creatures to live in the shallow ocean, but fast enough so the gills would change to lungs. Now animals haven't been on land for millions of years, so the land must be full of plants. Since the animals are so huge, they need lots of food so they won't starve and die. This need is met due to huge amounts of untouched plant life. Since the plant eaters are huge, their is no need for the predators to shrink. I know it is logical to think that evenually, over millions of years, plant life would run out, but it wouldn't. It is said that the climate back then was much like today's swamps and rain forests, and in these areas, plants grow like weeds. So since plant life is constantly on the rise, animals don't need to shrink to be filled up. Also, these creatures don't need to be warm-blooded, for the constant hot temperatures give these creature a good body teperature. So for years, these creatures were huge do to the fact that there is plenty of plant life to sustain these creatures. But then something happens. An extinction level event occurs to kill these creatures. This tosses massive amounts of dust into the air that takes years to settle. This kills an equally massive amount of plant life. Now, an ice age downs, bringing with it new creatures to populate the earth. These creatures are of course smaller, because there is little plant life that can survive in the freezing ice age. Also, the predetors are smaller. And of course, these creatures are warm-blooded since cold-blooded creatures would die. This is my theory as of why dinosaurs were so much bigger. Factoid #1- The Blue Whale is the biggest animal to ever exist, much bigger than any other dinosaur.

Actually the biggest animal earth has ever seen is alive and well today, No animal was or is bigger than the blue whale. Imagine the blue whale had to fly, a Jumbo Jet could only seat one whale lying down and still part of the whales flipper would hang out the end. Ironically the whale can only eat the smallest animals (about the size of a cell) in the world. Correction: the blue whale eats things bigger than a cell but if you stepped back to see the whole whale, you would not see the tiny creatures it is eating. As opposed to some large cats that can take down something larger than itself for a meal or two.

Well, scientists have come up with several reasons: They were cold-blooded, so they could be big enough to bask in the suns rays and not overheat. if you're wondering why reptiles today aren't that big, it's because mammals are now "the big boys" The biggest dinosaurs were sauropds, who had long necks and tiny heads. instead of the heart pumping a lot of blood a little way up, it pumped a little blood a long way up. I'm sure there are other reasons, but those are the ones I know of.

If species have evolved from cell organism. is it possible that any more new species would evolve from existing bacteria sort of creatures ?

They evolved into gigantic lizards due to an environment that favored this.