One option...and it's a compromise no doubt...is to buy a large pack but one which is light for its size and has many compression options (straps, elastic, removable 'day pack' sections, etc.). You'd still have the big maximum space, but wouldn't have to carry a big, heavy floppy, 1/2 empty pack.

The thing I hate most about carrying my 60L pack is the pack! Once it's on my back and strapped in it's fine even if overloaded. If it's not full enough, it's floppy and doesn't feel right. If it's overfull it's hard to get onto my back by myself...and there's no way I'll get any of that stuff out without taking it off again.

The day that I can go into the bush for a weekend with a small pack (35L or less) and still have everything I need, will be a VERY happy day...I'm working on it but still nowhere close.