I'm not sure. I never saw truly fat people in the infantry. Just guys who were somewhat fat but not full on planets.  The M16 has hardly any recoil. Pretty much any adult could learn how to control it so your second shot is still on target. Learning how to control a 3 round burst is tricky at first, but it doesn't take much strength. The SAW uses the same ammo as the M16. It just fires them a lot faster. The M60 has more of a kick, but we weren't running and shooting like in video games. Shooting from the prone position, your shoulder takes the force from the recoil. Even grenade launchers didn't have enough recoil to make size matter that much. We usually had bigger stronger guys carry the SAW or M60, but that's just from the weight of the weapon. Carrying an extra 20 pounds on a road march makes it harder. If you're already carrying extra weight from being fat, you'll have a hard time keeping up just with your M16 and regular gear.