I'm going to South America for a month and the 38L pack I've tried was a little overstuffed, with little room for things like souvenirs. I'm confused though because I found a 60L pack that was actually smaller in terms of inches. Anyone know how that's possible?

Also the airline told me 45" was the max overall size for a carry-on. So I'm also looking for suggestions for packs that are roughly 50-60L and smaller than 45" -- thanks!