I've used a ton of different gear over 34 years. When your young it's usually just what's cheapest and that's what it was for me. Medium Alice. Even young we knew where possible to stay with mil spec gear. Not so many aftermarket gear choices back in the 80's- it was either actual military issue or knockoff made in Taiwan. Lots of guys went to large Alice with frames. This made them carry too much, slowed them down and made them less effective.

You see this same thing in "preppers" now- buy a huge pack, put everything including the kitchen sink in it, put it on their shoulder once at the house then into a closet it goes.

You'll have no idea when the time comes if that will work if that's all you do.

We were constantly in the field, so gear was regularly changing. You got sick of the rubs in certain spots so you fixed them. You realized you didn't need 77 pairs of extra socks so you ditched them.

As you get older you seek more "snivel gear" to carry. The 500 rounds of extra ammo goes and a better sleeping bag and pad takes it place, lightening the pack a bit as well.

If you do this long enough and train regularly, your gear will be changing a lot.