I remember reading somewhere that inventory was based upon weight. Meaning larger guns take up more space than smaller ones would. So I would imagine that the heavier the armor you are using, the more you could carry, which would allow those people in MAXs to carry extra ammo for squad members. I would also guess that even in the light armor you would always be able to manage a knife/pistol and possibly 2 SMG types or 1 large gun(including all the ammo for them). Of course, this is all my guestimation
oh, btw, Hi! i'm new to the forums