That would limit the game's public Most of the game buying public don't give a shit if you can carry 20 or 200 shells, they'll probably think "Hey, that's irritating, I don't like that game!". So interplay would never implement it.
So you should do it like soldier of fortune, in which you can carry less if you select a higher difficulty.
Or you could make a low tech version of the car (or a bike or something) available early in the game, to expand your inventory.
By the way, I think the best way to use such a system is by using two variables. One for the size of your inventory which depends on your height, and one for the max weight you can carry, which depends on your strength. Cars and bikes should be using the same system. I think the car in FO2 has way too big an inventory.