Nope, it's the government's mileage standards. If you make a truck with a shorter wheelbase and track, it has to hit higher gas mileage standards. Easier to make a big truck that's allowed worse mileage.
Also, I did a brief stint selling cars in the 90s. One of the salesmen explained it like this, "What's the real difference in a big truck and a small truck? Same engineering effort, same production work, all that. Hell, same parts for most systems.
More steel on the big one, and steel is cheap. We can charge a premium for the larger truck."