I think you're over-analyzing the subject.

In my view, packs are somewhat over-rated. A load is a load and not good, whatever it weighs and however you carry it.

Frames are fine, design is fine...........and these help a little, but won't lighten the load.

Big packs are easy to pack.... and therefore offer some virtue.......But the best pack is always the smallest size you can deal with.

Bigger packs will mess you up, more or less.............whether they are fully or partly loaded. So will small packs, but a bit less than big packs.

Fit is another question and more easily addressed.

It's always best if the pack fits, but even this, obviously, won't change its weight.............