Mary is very fit and has a low body fat level. Perhaps she can't go much bigger because there is simply not enough fatty tissue in her breasts to make it possible to do so. I look at Foxy, Allegra, Chelsea and Beshine and they are huge but none of them are anywhere as fit and lean as Mary. No, they are not fat by any means but they seem to be able to expand more easily because they are not so lean. Of course, every individual has a unique physiology which allows the body to change, or not, so we just have to wait and see.