Brewtal, DoomRL can't be like original Doom. What would you say if player could have only 200 or 300 ammo for chaingun, no medkits nor armors to carry and so? Everything comes for a price. It's a roguelike so it has no real-time and it'd be too easy if you were fully armed and equiped walking tank. I think the size of current pack is balanced. Try to finish the game on AoLT challenge and then you'll know that it's huge, actually :)
I think that adding a backpack (as in Doom, Quake and so on) is a good addition. It'll be some (unique) item or a trait perhaps? Porter, master packager and beast of burden hehe :>