If your bag is almost full and you cant pick up the full amount on the floor, from the inventory, if you right click the item, it will pick up the max that can fit into your bag.
From personal experience, I don't keep more than 150-180 rounds of primary ammo in my bag, 50 secondary ammo, max of 10 bandages and 2-3 grenades. The rest is for meds. I've seen people with 66 bandages and 450 rounds, taking up a shed load of room!
You can pick up packs of 30 ammo from the guys you kill. You won't need to spend 10 mins looting the guy when your bag is half empty too, same goes for swapping your weapons at loot crates.