OK, I'll try to argue that big isn't always better. As I've said, I like a pack that is a little bigger than I absolutely need; it give me some options and still rides comfortably.

However, I don't like large for the sake of large - and a pack can be too large. With a lightweight load, significant over-capacity can lead to no load control. For example, I tried a Gossamer Gear "original" pack (I forget what it was called - G1?, G2?), and found that it was simply too large. I'd put my sleeping bag in it, totally uncompressed, then load in everything else - which compressed the sleeping bag, and let the load go too low. I never could get that pack to fit right, even with all the compression system fully tightened. With no load control, and a load riding too low, my shoulders ached and it felt like I was carrying a sack of flour around.

In short, "big" and "small" isn't an argument that can occur in a vacuum: you need a pack that fits the load you plan to carry most of the time without forcibly compressing it to the point you're carrying a stiff pack. It needs to provide sufficient load control (yes, Jim, even at 20 pounds, I need load control - not as much as with 40 pounds, but still some control.) A bit larger pack than you need provides that little extra capacity for that once-every-two-year two-week trip you take, or when you visit that beautiful, but water-scarce hideaway and have to carry more water than usual.