I don't have a large bust but they are still the same size although I don't think they are droppy. It is true that they do get huge when you are preganat (which is quite a nice side effect) so they are bound to be a bit disappointing afterwards.
I have breast fed five babies now (the last two are twins) and I would wholly recommend it - not from some earth mother, knit your own underwear point of view (even though I know the benefits) but it is so convenient! On my first three children I had to return to work so they had to go on to bottles and it is such a fuss - sterilising, making sure you have enough formula with you (and holidays are a nightmare for that), is it too hot/cold and it is really expensive.
(PS - you'll be so knackered for a couple of weeks that you won't care what you look like but on the plus side you'll have one less thing to do if you don't have to sort out bottles!)