If i remember right.. mine went from being firm to quite empty looking around a week after jacob was born (never breast fed) then about a month on they firmed up again. I'd say i'm a tiny bit smaller than i was before jacob. I wear the same size bra as i used to.. just feel i don't fill it as before.
To be honest mine don't seem to have changed at all during the pregnancy nor afterwards.....they feel fuller, but they certainly don't look it (but I am not small chested anyway) so I have no advice here, sorry!