I pack a 10 liter MSR dromedary bag, a two liter dromedary bag, and four and a half liters water bottle capacity. I put the big 10 liter pig - 18 pounds of water! - strapped to top of back rack when full.

I've been making the switch to Platypus bags, nice and compact when not in use. the Dromedary bags only slightly less so.

I carried a gallon on an ultralite trip this summer in the Marin Headlands by slinging 2- one liter Platypus bags off the back of a Carradice and two liters in the bottle cages.