I am curious to know just how much genetics plays into it.. Obviously the younger that you start hormones the more of an effect there will be, but for someone who's mother is an F, sister is GG and grandmother is an F... well it would be rather annoying if I stopped at A-B when I'm starting at 21.
I haven't actually seen a case of significant growth yet, only in the form of implants.
But anyways, back on topic.. if you're worried about them being visible then just a sports bra or even a binder would do it.
In the FTM section its really surprising just how well the binders work.