Men on the internet also seem to be under the impression that the only way boobs ever grow after high school is because somebody got surgery. Boobs get bigger for many women for all sorts of reasons all the time. Weight gain, pregnancy, hormonal changes, birth control, switching types of birth control, diet changes, exercise changes, medications, menopause, and all sorts of other reasons can cause boobs to get bigger.
I don't know anything about belle, but any girl in her early 20s could easily still be growing naturally.