With a larger pack, as long as it doesn't have big 'bellows' type pockets on the outside you can always fit compression straps to compress it right down. I look for lid straps that come from the bottom of the pack, so you can cinch the lid right down without running out of strap, and side compression straps that can basically haul the pack main section in until it's flat. That way you can have a good amount of 'reserve' capacity without the bulk when you don't need it...
Not the easiest thing to find, but some of the older Macpac and Fairydown packs had this sort of compression system and it was very good. My 85Lt full size Macpac can crunch in smaller than my 40Lt 'overnighter' is this fashion. Downside of the older packs, is that while they were and still are literally bullet proof - they are bloody heavy and noisy by today's standards.
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