Fallout already has a weight limit for equipment. Adding restrictions to bulk size like in Diablo (for example) is rather annoying in practice. As far as I know most (all?) PNP RPGs don't impose such restrictions, not only because it'd be tedious to keep track of (this could be solved by using size values (analogous to weight) rather than the tetris-like system Diablo or Deus Ex use), but rather because it's more of an annoyance than an improvement.
Realism rarely equals fun -- having to carry supplies around when travelling may make a game more challenging, having to carry supplies around to live another day may ruin it (if that's not supposed to be a core aspect of playing the game), having to use a toilet regularly and not forget to wipe WILL ruin it (unless your game is The Sims or some kind of toilet-business simulator). The same applies to inventory restrictions.
Fallout's invisible supersized backpack may not be realistic, but it's pretty much what a PNP RPG player would expect -- and Fallout is all about capturing the PNP experience.