More mags! SAW pouches are a perfectly legitimate way of carrying magazines, they are designed to take 6 x M4 mags, plenty of US personnel used them as such
Anyway if you think that soldiers will waste any space on their vest for more ammo carrying capacity you are mistaken Skeletor, Just look at the photography book Whisky Tango Foxtrot (or any other Iraq invasion photos for that matter) for plenty of examples of mag pouches spread along the whole length of the vest.
They aint going to leave an empty space for more bullets just because its "awkward" getting a mag out (these guys aren't playing a game of airsoft where you just need a couple of hi-caps to last a few hours for a game, they need mostly ammo and water for foot patrols).
Great pic 5kull, from a great movie!
(Just realised the pic has Matt Damon with mag pouches on his left side skeletor