I have a Catalyst with 2 water bottle holders. I found the best size for me is two, 24 oz Propel bottles. The 32 oz are pushing the limit for staying on in rougher terrain or when moving quickly. These sizes don't slap back and forth for me, or slip out of the bottom. They are still an easy enough size to pull out.
I mention this because 'it seems' like having this extra 3 pounds on the front of the pack counterbalances some of the load. For myself, this is in lieu of putting the same weight in the side pockets of the pack. This would push the center of gravity of the pack further back, requiring the hiker to lean a little more forward.