I think it is because the rear drives the vehicle and requires a significant amount of traction considering the heavy load it need to haul. The front is for steering the vehicle. I did not look this up but it makes snese to me. There may be more to it though.

A quick search on the net found no really good answer to this question, but a few theories. Here is a good one: http://www.newton.dep.anl.gov/askasci/eng99/eng99307.htm Hellaphunt's answer pointed out that one of the aspects of tractor function is that the large rear wheels do the grunt work and the larger they are, the less likely they will be to get bogged down on a wet field; there is no need for the front wheels to be as large. Small front wheels give the tractor operator a clearer view of the rows through which it is working and possibly better turning capability. As the link says, large wheels are harder to steer, especially in the tight turns necessary in working a small field. In the days before power-assisted steering; this would have been a large factor in determining what size to make the front wheels. You can still find very old tractors with tiny front wheels very close together - they would be more susceptible to roll-over, so I would recommend staying away from those if you are considering getting a used one. Today's tractors come with wheels of many sizes, even some with all wheels the same size. Four-wheel-drive tractors are also available with power-assisted steering, and air-conditioned cabs, not to mentions ROPS (rollover protection structures.) Old MacDonald would be envious!!

The rear wheels are large to harness the immense amount of power generated by the tractor. In addition, fields are not always ideal for driving so tractor tires have huge treads to really grip the ground and keep moving. The last thing a farmer wants is to have a $200,000 tractor stuck in his field.

According to Newton's 3rd Law of Motion, for every action there is an equal and opposite reaction. The large wheels in the back push a stronger force toward the ground making it easier for the tractor to go through dirt, rocks, etc. . . . Also the front wheels, being smaller, allow the larger wheels to provide a diagonal force toward the ground making it easier to get out of ditches.

On any vehicles, the bigger wheels give you more power, while smaller wheels give more speed. It is a gear ratio thing. If you look at old fashioned steam locomotives, the freight engines have bigger wheels and the passenger locomotives have smaller wheels for speed. During the last fuel crisis, the automakers put tiny wheels on some of their cars to get more cruising speed out of them while reducing friction and power, thus getting better fuel economy. http://www.escortfocus.com/html/history.html Those big wheels on the back of a tractor produce so much power that they easily and frequently lift the front wheels off the ground. If you ask too much of them or try to go over a bank with them, those front wheels can come up enough to roll over backward. Many tractors carry weights on the front end for just that reason. Another reason for using larger wheels on a tractor is that it gives the tractor the ability to go slower. Many operations done with a tractor involve a human element as well. A transplanter, for instance, is ground driven and mounted on the tractor, but it requires people to sit on it and feed plants into the machine. If the tractor goes too fast, the people won't be able to keep up with feeding plants into the machine and it will leave long gaps in the row that must be filled in by hand or weeds will take that stretch. By having those big back wheels, you can gear the machine down to where it barely moves at all, without stalling the engine. http://ces.ca.uky.edu/owen/anr/Tobacco/PhotoEssay6.htm