Why not tie it into the comics code idea and have every time she does something she shouldn't- such as curse- she expands. The more extreme her danger, the more extreme her reaction, and therefore the more extreme her bust.
Think of it as if every time Robin actually said to Batman, "Holy (
https://en.wikipedia.org/wiki/List_o...tions_by_Robin) Batman!" She would actually use a more creative and pure expletive and therefore be "punished" by the comics code with a bustline to rival Pinocchio's nose.