I hate big packs.

I put all my gear in, and they sag and require lots of straps to compress the sack down to a not-floppy state.

And when I'm out with a pack of newbies, and they find that they packed too much and have stuff dangling off their packs, and start dragging and sighing that it's too hard, they're tired, etc. - some bright person notes that I have plenty of room in mine!

It's far too easy to put a cheesecake in a big gap in the pack! carefully packaged in something to keep it from being crushed, of course.

But in all seriousness, I believe in load balancing and minimizing bulk, because I have to ruck the thing all over the mountain, through tangles of trees and brush, and get it in a very small helicopter along with someone else, their pack, and maybe even their search dog, some of the time. And those are things I cannot do with a massive pack frame.

There are times you can get away with a pickup truck of a pack, and then there are times you want something smaller. I could not fit a huge pack inside my very light, solo tent for example, with room for me and the rest of the gear - and in Yosemite leaving the pack outside is an invitation to the pack stealing bears.