Rear in vehicles means the size of the rear wheels is larger than the size of the next wheels because the weight of the vehicle and the people sitting on it or in it is more on the rear wheels than on the next wheels. Also, the rear or rear wheel receives energy from the engine directly on the torso.
And is responsible for driving the vehicle while the front wheel only moves the rear wheel and is used to guide the vehicle. Rare wheel friction is much higher than the road, it also wears out more quickly. Due to all these reasons, the size of the rear wheels is large.
f the tractor engine is weak then how does it carry more burden This is because - the tractor - produces more traction or torque - losing its speed capability which is only one-sixth of the speed of the car. In simple words, the strength of the engine is spent in carrying the burden, not in the speed.
That is why the ratio of speed reduction in a tractor is 40 times that of a car. Any vehicle, whether it is a road driven or a rail driven engine - its designers decide whether to produce more traction/torque or higher speed.