In reply to CurlyStevo:
> (In reply to lost1977)
> thing is smaller/lighter packs are normally less comfortable than larger/heavier ones when the smaller/lighter one is overloaded.
Which is why I prefer larger, lighter packs these days. Especially my GoLite Jam2 and Pinnacle, which are 50 and 70 litres respectively, weigh next to nothing and carry better than my disappointing Macpac Ascent (which looks nice, has bombproof fabric, got great reviews, but is fatally flawed by straps that keep slowly loosening all the time). But I tend to use the 50 litre pack for summer climbing or winter walking, and the 70 litre for backpacking or winter climbing, and neither ever needs to be shapeless when half-packed or less because not only do you get the usual side-compression straps but you can also effectively clip out the bottom panel. They're quick to get into (with no silly lid or a thousand buckles and drawcords to get in the way when one will do), surprisingly robust, altogether a revelation, and only inferior to the Macpac IMHO in not keeping loose contents dry in a downpour (which is why I have dry bags for things that matter).
Might add that my smallest sack (apart from specialist running sacs) is now 50 litres and my backpacking gear all modern, light and compact, but I still prefer the larger sack for many applications. So stick to 20 litres, or 30, or 40 for winter if that's what you prefer, but I wouldn't!