My boobs got a little bit smaller when I went off the pill, but not even enough to drop a cup size.
As for during pregnancy... with DS1 my boobs went from a DD to an E by 6 weeks and were an F by 20 weeks. They got up to a G when my milk came in, but settled back to an F after a few weeks, an E after a few months and back to a DD when I stopped BFing.
The second time round they didn't get quite so big and have settled faster.

You are likely to go up a size or 2 at some point. If you don't want to be bigger than a FF then losing some weight is probably a good idea, though it may not come off your chest. A breast reduction is major surgery though, so I would be trying every other option first.
On the plus side they make far more maternity bras in large sizes now. I had trouble finding them for my first pregnancy, but have come across plenty this time.