Why are the rear tires of the tractors bigger than the front tires?

As the traction comes from the rear tyres, it's important to ensure that they have enough surface area to gain sufficient traction, and also enough surface area making contact with the ground that it lowers the ground pressure to a point where the tractor won't sink into the mud. Since the front tyres are the ones which actually turn the vehicle, they're smaller to ensure that they can turn and have enough clearance where they won't rub against the frame of the tractor.