I have a friend who gained something like three cup sizes in her early twenties. She had an eating disorder in her teens, got better, gained a little weight and went from A-cups to D-cups. I think it seems plausible that you might see an increase if you eat better and fill out a little.
Breasts aren't ever stable or constant in size, in my experience. I've always had large ones, but went from large C-cups to E-cups a few years ago - around 25, that was. I gained some weight, most of it ended up in my boobs, then I got rid of most of the extra flubber, but I'm still stuck with DD/E, unfortunately.