To add, practice the key limit on capacity rather than body size. I'd actually argue that heavier/larger folk will have a bigger capacity, just to do with sheer mass.
I think the reason why people think that being fat means you don't get as big, is because when people start inflating they will have a smaller capacity. It might be more visable on thinner folk, but even then, a smaller person will still have a small capacity due to practice.