It IS, indeed, possible for a woman to 'expand' this much, hormonally. What you must remember, however, is that such growth is rapid (relatively speaking), and so leaves it fuller and firmer then it would be otherwise.
Consider this - Compare a pre-pubescent girl's bust-size with a post-pubescent girl's. That's hormonal, over the same amount of time, and with the same general increase, overall. The women who go through this are simply going through a sort of 'second puberty' from an out-of-whack genetic marker that kicks in.
After all, as R.A.H. pointed out, it's easier to work out the math describing a genetic set-up that says 'start doing this, and keep doing this' then it is for one that says 'do this JUST ONCE, at a SPECIFIC TIME, then STOP! - and never do it again...'
Gunny