There's still CAFE (corporate average fuel economy) calculations that have to do with the "footprint" (wheelbase x track width, as I understand it), and the vehicle is exempt from CAFE as a whole if the GVWR is more than 8500 pounds. It leads to manufacturers gaming the system, either by putting vehicles past 8500 lbs GVWR, or by using longer wheelbases so that it's allowed to use more fuel.
The "150" or "1500" badged trucks (depending if it's Ford or anyone else) are below 8500 lb GVWR, so CAFE applies, and my understanding is that this had to do with the disappearance of the short-wheelbase, standard-cab short-box configuration in the fullsize trucks, and the near-disappearance of compact trucks as a whole.
My van is badged as a "1500" but it actually has a load capacity inside the van of almost 2 tons, because they wanted GVWR to be just over 8500 lbs (it's 8550) so that CAFE didn't apply ... there's no reason for it otherwise. I'll never use that much capacity, and I downgraded the rear springs (took a leaf out) so that it will ride better and sit lower ... that's more useful to me than the extra load capacity. There's little difference between a badged "1500" and a badged "2500" because of this.
The US has some crazy depreciation rule that applies to higher-capacity trucks used for business purposes - something to do with being allowed to write off the entire cost in the first year. Buying a company truck? Might as well buy a heavy-duty expensive one.