I use a Stone Glacier 7400. It can be adjusted from approx 30 litres all the way to 120 litres. Weighs about 2kgs and the external frame is strong enough to carry out most of the meat I'd get from a mature hind/spiker.
It's handy having that kind of flexibility in a pack. If it's a day mission I simply compress it. If it's a multi week trip I may fill the entire pack.
I've found that it's always better to have extra room in a pack and not need it. Besides, filling space does not necessarily mean extra weight. Some items may be 'ultralight' but bulky and require extra room. E.g a 2nd pair of boots for once you hit the snowline...or a big puffer jacket.
Sometimes I may not compress my sleeping bag in its stuff sack and just let it fill extra space in my pack. This approach can increase the longevity of a down bag. Or I may get into some thick scrub and the extra room in my pack will allow me to move gear from outside of my pack (e.g tent) into my pack to keep it from getting torn.
Some good manufacturers of compressible packs include Stone Glacier, Kifaru, Mystery Ranch and Exo Mountain Gear.
Beats carrying two seperate packs, but they're not exactly cheap either.
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