Briefly explain why the tyres of tractor are large and wide?
Large and wide tyres have a larger surface area. Pressure is inversely proportional to surface area. So for a given force the pressure will be reduced with a larger surface area.
Reducing pressure reduces the damage that the wheels do to the ground. Thus the amount that the wheels sink into the ground will be reduced. For a tractor this is useful because they are often driven over soft ground.