Why are the rear tires of a tractor bigger than the front tires?
Torque and angular speed of axle are not decided by size of tire, but by engine. Although M.I. of the tire contributes its effects on torque and angular speed, it is not significantly. So tires can only control traction force. Tractor is supposed to run on soft terrain like farm where coefficient of friction is comparatively low. So there is a engineering challenge of less available traction force. If a smaller rear wheel is used then for a particular torque available from engine it can produce higher traction force (Torque = Force x radius) but only on the terrain where sufficient coefficient of friction is available, and if terrain is soft and we try to produce high traction by reducing size of tire, the tractor will skid, which is undesired. That is why larger rear wheels are used in tractors.