I agree with what's been said- water capacity is one factor among many in a particular model's design. The manufacturer should have that worked out during the development stage of the unit. I guess I don't see low or high volume as good or bad, just different. Kind of like a flywheel, sometimes you need inertia, sometimes you don't.
I often get the "how many gallons does it hold" question at shows, especially from guys with high volume units. Some NC manufacturers near us went high volume to accommodate solar due to a state incentive. Now some see it as a measure of efficiency or btu output, which it isn't, just ability to store heat.
Generally speaking I would prefer quick recovery/low mass but some situations would do better with higher mass, like greenhouses or other big fluctuating loads. For most residential installs low mass works well.
Just my opinion.