>>2479933One person's base gear and consumables combined can easily be 50% bulkier than another's, and trail topology, calorie intake, water needs, weather, and resupply frequency (if any) can vary widely, so there's no hard and fast rule, just a vague sliding scale.
Also, some bags can and do pack more efficiently than others, and different people are also better or worse at packing. As you can see, it's a whole mess.
For you, for two days to a week, I'd say stick with a 55ishL pack that can easily expand and/or have stuff securely lashed to the outside (via MOLLE/daisy chains, attachment points, etc; some packs will fit one or more expansion pouches), figure out what you don't really need, and also maybe see if you can pack a bit better. My 55L is plenty for me for that same period of 2 days to a week.
Because having a snugly-packed pack is important for balance/load distribution/center of gravity, etc., so just having something fuckhuge that's loosely packed on most outings is not a good option. You will almost certainly have your sleeping bag at the bottom in a compartment or bottom rigging, your tent also at or near the bottom in compression straps; get your sleeping pad and/or air foam mattress onto there through whatever suitable means, and lash or jerry rig on a pouch if there's not enough room in the main compartment for all your shit.
