Automobile Engineering Question:

Answer #1
Tractors have big rear tires because ,the larger the surface
area of the tractor's wheels touching the ground, the more
pulling power you have. Like how tractor pullers have large
tires.

Another reason is that the tractor doesn't have to operate
at a higher speed to get the same ground speed with larger
tires. Gear ratios don't have to be as high b/c of the
larger tires.

Answer #2
Generally tractor is used for carrying heavy material. & for travelling the gradient more torque is require, we know torque is directly proportional to diameter, that's why big tyre is used.