What's really interesting is that the bed size on both of those trucks appears to be the same.
For myself, when I'm looking at pickup trucks--which I sometimes do, although I am unlikely to buy one any time soon--I'm looking at the ability to carry full sheets of plywood.
A lot of the older trucks--like the early 80s F150s--were veyr small by current standards, but still had the bed size and overall hauling capacity of current gen F150 trucks. They were very clearly work trucks though, and had very few comforts that you'd expect in a car.
the bed size on both of those trucks appears to be the same.
The length, definitely. But having owned similar sizes with the same bed length (Mazda B2200 & F-150), I can tell you that the larger truck has a much higher max weight capacity, with at least 2-6in greater rail height and anywhere from 12-24 inches greater bed width.
As in, you could put a standard 4×8 sheet of plywood down in each, but in the smaller truck this sheet would be sitting on top of the wheel wells, while in the larger truck it would likely be sitting between the wheel wells entirely.
I get what you're saying, but looking at old full-sized pickups, the overall width is about the same. What's different is the ride height and wheel/tire size. A very high weight capacity creates it's own set of problems, namely that control is compromised when you have no load at all, because there's no preload on the springs.