>>194546well, the big packs are designed for week long treks and multinight trips, and as such they have a bottom compartment dedicated for your sleeping bag, and they also have much burlier straps and hardware.
The smaller packs are designed for people who are going out for the day and are not taking a sleep system with them, they have lighter straps and hardware and are lighter overall.
If you were to pack one of the larger packs without a sleep system, the weight distribution will be thrown off and it would become uncomfortable. That, and a proper hiking pack will be designed with compression straps to keep the load from shifting, and if you pack your large pack without a sleep system, you probably won't be able to get it compressed properly, resulting in a floppy pack.
That's not to say that thou shalt not use a full size pack as a daypack, but I had the funds available and really am a raging gearfaggot.
