If you can't look at an F150 and an F250 sitting side by side, each with 2000lbs in the bed, and see why putting more weight one would not be a good idea, and putting more weight in the other is acceptable, I don't know what else to tell you, but I'm gonna try...
You can tell an F150 has about all it wants to handle with 2000lbs on the bed. The F250 isn't even squatting.
Not convinced? Take them for a drive. You will want to park the F150 after about 10 miles, but you would have no issues driving the F250 all day.
The F150's payload rating is to get a stack of lumber home from the big box store. The F250's payload rating is for working all day every day. Yes, the manufacturer does not say that, but common sense will tell you this is true.
Of course the question is why do you see F250s with 5th wheels all the time, right? Well the answer there is because the truck salesman told them it could tow anything, and RV salesman agreed. Luckily the trucks are VERY underrated.