Farm lands and fields are not smooth and hard like play grounds, and so the steering effort needed to be put in by a driver has to be more to drive a vehicle in them.
In case of tractors, the front wheels are meant for stability and steering only. They are small in size to aid in steering and to give a better view of the field to the driver. But the big rear wheels have specially designed tyres for an entirely different purpose – to give a better grip on the ground. Their larger surface areas in conjunction with a low air pressure (0.8 – 1.2 kg per square cm) ensure adequate contact area (and friction) between the tyre and the ground. This is necessary to increase the driving force in muddy fields.
If rear wheels are also small in diameter and width, their contact area with the soil surface will be small. This may, if the field is muddy, make the wheels penetrate and get bogged down in the mud, due to its own weight. If the wheels are large this cannot happen as a considerable amount of sand beneath the tyres has to be disturbed.