Well, the word "most" is kind of loaded here since
its not really known if mtf's breasts grow as big
as their genetic potential.
But, my opinion is that mtf, if properly treated, and not
too old, can reach their genetic breast size potential.
If consider than the main difference between
transitioning earlier and later, when considering
the breasts, is HGH (human growth hormone).
With age, this factor decreases, but the speed of the decrease
and is highly variable. With a lower HGH, breast growth
will be slower, yet should reach its genetic potential eventually
if a decent feminising dosage of hormones is taken for the
entire period no matter how long it is.
Say, at puberty, 2 year to get full devellopment, could become
5 years at 40, for some people (for others, it could be 2, or 8 ).
Past a certain point (different for every person), the age related HGH decrease, impact breast
growth rate so much that it might never reach its potential before death.
This happens in very high proportion in older TS's,
say those of 60 or more, but could happen earlier too.
Anyway, in my opinion, if your under 40 and under a decent feminising regime, you should be able to
to reach most of your genetic potential, whatever it may be, inside 5 years.
That being said, some are not patient, don't like the hand genetics, or in the case of
older TS's, are not ready to wait 15 years taking a high dose of hormones to achieve
full growth, so they go for a breast implant.
It is not recommended though to do it before having waited 2 years for hormones to work (and as I
said, in older patients, you may be surprised by the growth you get after that on top of your implant).
Myself, I've had 36C in 11 months at 39 years old. Not a typical result, but not that exceptional.