I have a Topeak seatpack that's HUGE, I think that if I overextend it it can get up to 2 liters in volume. I keep one spare tube, two CO2 carts + adapter, 1 bar, 1 gel, repair kit, Topeak Alien tool, and there's still room for my bunched up jacket in there (ok it's very light and packable). Or my digital camera (which is on the big side, 4 AA batteries in it). I have needed all of that stuff on my ride today (plus I could have used a spare TIRE), I was glad to have it. Riding alone a lot, I wouldn't want to be stuck without at least a tube and some way to inflate it.

(Cause in my jersey pockets I already have more food, tissues (for when snot rockets just don't do it or do not seem appropriate), ID, arm warmers, etc.)

I love it, and I have never touched it while riding, but maybe that's just my bike's configuration...