If originals were in good condition but too heavy to carry, then the owners who modified them (due to weight) should have hit the barbells a bit more often, to strengthen themselves.
Our young skinny soldiers in WW2 managed to carry a heavier gun and a backpack for many miles per day.
13th warrior (when handed a large Viking sword): "I cannot lift this!"
Viking (laughing): "Grow stronger!"
Yes, our skinny soldiers carried a heavy weapon and pack many miles a day, because they had no choice in the matter! I've seen 5'2" 140lb guys humping a BAR or an M60 machinegun. It not that it not do able, its not the same thing.
Military rifles from the last century were built for combat, not just shooting. And combat included hand to hand combat. Bayonet lunges, and using the rifle as a club, or to block one being used against you. Compared to sporting rifles, they are massively overbuilt.
Why would anyone out hunting want to carry a 9lb rifle, when the same rifle could be trimmed down to 7lbs or so? One thing you can be certain of, when you see a altered milsurp, either Bubba botched or exquisitely done, you can be sure someone used that rifle for something the military did not. (hunting, usually)