I am not an engineer, but I will guess why this is so while thinking about my car. Chapa is an engineer and he may have a better explanation. I will give it a shot though.
If you look at the tractor, and other vehicles, the front wheels are the ones controlled by the steering wheel. So they are important in making manouvres. As you can imagine, it is easier doing this with smaller wheels. However, the tractor is meant to move heavy stuff through huge force/torque. This is where the rear huge wheels come in. The can traverse rough terrain and help push the tractor through muddy and rocky grounds.