Big packs are great for winter and very long trips. If you can, take all of your gear with you when you look at packs and see what fits. I took a fairly small pack to NZ years ago (a North Face Moraine, 4400 ci/72L) on an extended trip and it was fine. My bag was a Cat's Meow which took up a fair amount of space.

The ultimate answer is "it depends." If I was replacing my current pack, I'd buy the big one, but I winter camp, so the extra space would come in handy (my parka packs down to the size of a sleeping bag, so it's like carrying two bags).
Don't get me started, you know how I get.