Why Rear Wheels are Bigger in Tractor ?? || Tractors are generally employed in muddy ...

... fields and off road conditions. Name tractor itself comes from the word traction. Most of the power and traction is required for rear wheels in order to pull itself out of such muddy and uneven fields , so all tractors use Rear wheel drive mechanism.

For more detailed understanding , let's take a example of a tractor with a ploughing trailer. When the field is ploughed, the pressure is more on the rear wheels than front wheels because of the digging of field with trailer which is attached to rear of the tractor.

Remember this two fundamental formulas ,then one can understand anything very easily
We know,
Pressure = Force/Area (P=F/A)

Power = Torque * Angular velocity (p= T*w)
Reasons for a larger footprint and larger diameter of rear wheels using above formulas:

A tyre having larger footprint(More area) will have lesser pressure acting on it and it will be able it pull its weight easily out of mud.

For a tractor moving in the field with certain velocity the front wheels spin faster than rear wheels because radius of front wheels is smaller than that of rear wheels, this means for the same power rear wheels will have more torque capability than front wheels which is an advantage to come out of uneven and muddy fields easily.
*Front wheels has got no reason to be smaller, but smaller wheels reduces the steering effort . So generally front wheels are smaller.
Please comment if you like to add to discussion . Thank You!