"The marine environment allows an animal, such as the Blue Whale, to grow large. This is primarily because of the buoyancy aspect. The water will support a large-bodied animal whereas on land the skeletal structure must totally support the body. In the latter case, gravity doesn't allow such large form to be so functional. The structure of cetaceans in general, reflects a completely aqautic mode of life. The body is fusiform (cigar-shaped). Most vertebrae have high neural spines and the cervical vertebrae are highly compressed. Little movement is possible between the joints surrounding the shoulders. In other words, the support is from the environment and the phylum and specific species have adapted to it. From an evolutionary standpoint, a terrestrial animal that became aquatic might grow large and as it browsed in shallow seas, could maintain enough of a food supply to support such a large body mass. Some archeocetes grew to large sizes. We know this based on fossil sizes. In colder waters it would also adapt to a large sizeas the result of gaining the blubber sheath in an effort to maintain homeostasis. Large size also allows them to fast during calving times (when they esentially don't eat!). Large size allows for deeper dive capability without suffering diving (decompresssion) problems such as the "bends." Peter M."Skip" Scheifele Director of Bioacoustic Research, University of Connecticut

Here's a question that I've never seen any answer to. Whales are bigger than any animal that has ever lived. The largest dinosaurs were maybe 50 tons, whales can fairly easily (if they're allowed) reach 100 tons and the heaviest estimated whale was over 200 tons. No whale ancestor in the dinosaur days was ever any where near the size of modern whales. So in our period in history, we have the largest creatures ever - by far. Why do the whales grow so large? There are plenty of examples in the fossil record that in a particular animal line - take dinosaurs as the best known example - there is a tendency to gigantism. In other words if all other things remain equal, then through evolutionary time, the largest animals will prevail as they are the most efficient at obtaining food, breeding etc. The danger is when change occurs, large animals can't adapt as well or as quickly as smaller animals and tend to be the first to die out. The change as far as the whales was concerned was the advent of man the super-predator. Now some 30 years after most commercial whaling has stopped, the larger species are still no where near recovered, whereas the smaller minke whales for instance have increased enormously in number to the point where pressure for a sustainable fishery is increasing. Why are whales so big? Why are they so big now? - more so than any other creatures ever have been when the advantages of large size have always been there and the possibility of growing large always existed.