Most Helpful Girl
Boobs can always change size because of diet, hormone changes, stress, genes, etc. The rate of how they change can be different too since personally, I could fit in a training bra until I was 18, I grew sort of into the B's, then my boyfriend started to grope my chest and they were growing into C's in my 20's.
From the Japanese myth, if a guy gropes the female a lot when they're young, the girl will have big boobs. However though they do grow bigger, it sometimes is not a large extent or are most of the time temporary because boobs tend to be bigger during/for intercourse. The exercise though can make the enlarged size permanent for some girls.
The tightness of clothing do not affect the size of breasts, though it does affect your muscles or shape of them. There were discussions that there are herbs to make them grow bigger, but I find it unhealthy or still a myth since it was not in actual articles, the discussion never said what plants they were, and there are no research or people with medical degrees to be validating the existence of boob enlarging plants. I'd just say drink lots of milk, get massaged a lot, don't fret about the size because some boobs look better smaller than bigger. Like they start sagging if it gets bigger side effects or what not.