The rear wheel of the tractor is the one which derives the engine power. Tractors are supposed to be used in farms where the soil is loose. So in order to avoid getting sunk in the soil, the rear wheels have large surface area (p=f/a). Moreover, large radius of wheel ensures more torque which is also required. Lastly, bigger wheel provides more traction. The front wheel is small since it is for steering purpose, and small wheels ensure easy steering.