However heroic you may be, your crewman feeding ammo is not. Only the top two cards matter. They get the penalty for moving, but still retain the benefit of the cover. I set up US vs Germany, giving the US 3 Groups of 4 and the germans 2 groups of 5. The germans kept winning, because the US never seemed to have enough firepower.
Firepower is what large groups are good at, but large groups also have some serious weaknesses.
Probably the most obvious is that a single pinned man in a large group prevents that group from moving, and rally cards of sufficient size are not always handy. This is especially important for the Germans since they can discard only one card per turn. Use the fire group to suppress enemy fire teams while the manuver groups burn movement cards with lateral movement untill you get lucky enough to pin a bunch of his shooters. Then rush forward while the enemy rallies and nab yer good terrain. Large groups have better firepower, but they are also bigger targets, since one fire attack attacks all men in a group.