Well I'm a smaller guy, and even when I did go to the gym 4-5 times a week I was never very strong.
Playing an outdoor game recently really opened my eyes to how light a gun feels when you pick it up, attach it to a sling and start walking.
But after keeping it in a ready position for awhile and trying to fight through the brush a heavy metal gun gets tiring.
Doing a patrol with an MP5 or a compact M4 sounds pretty sweet.
Hell, 1 CQB game I was assigned a window to cover and having my pistol up in firing position for 3-5 minutes straight felt like pure torture.....but I digress.