Question

Why are the rear wheels in a truck are more broad than the front wheels?

Answer 1

it's actually not that common but the REASON behind it is usually traction or load carrying ability. Wider tires tend to achieve more traction than narrow, which is important on a rear wheel drive vehicle, and if it's a sporty truck/SUV, it's partly for aesthetics too to look more sports oriented. 
If you're referring to trucks with dual rear wheels, that's typically just to handle heavier loads...

Answer 2

Answer:

As you might know, the wider the area, the less the pressure. As the load is kept behind the truck, The wheels are more broader. The area decreeses the pressure taken on by the Truck.