What's the Benefit of a Lightweight Rifle?

I spent my first year of active duty in the Army as a Private
E-nothing 11B10, humping the M-14 through BCT, AIT and OCS. That was a superb rifle for long-range shooting and for Drill and Ceremonies, but it sure got heavy as each day wore on. Even the ammunition was heavy, when we crammed 20 rounds into a magazine and carried at least 10 full magazines when we went to the field. When I got to ‘Nam and was issued an M-16, I thought I had died and gone to heaven! The difference was incredible! Not only was the rifle half (almost) the weight of the M-14, but the 5.56 mm ammo was so much lighter than the 7.62 mm rounds, that I could carry four times the number of rounds. And I didn’t need the long reach of the M-14, since my engagements were always less than 300 meters. I was grateful for the reduced weight and ease of handling with the M-16, and if I had been given the choice of which weapon to fight with, I would have always chosen the M-16 (the variant was actually the M-16A1).

Size does matter. A person who weighs in at 90 to 125 pounds or less can handle the smaller rifles without difficulty. The larger the person the better ability to handle larger weapons.

Bottom line benefit is ease of carry. Going to have a long trek to get to where the game is? Go light.
Need stability in fire mode? Go heavy.
Light rifles of any given caliber recoil harder because they have less weight to overcome in recoil. You will take the difference. Or your shoulder will.