Years ago I was in PT for my back and they spent a year trying to strengthen my
core in an effort to help my
back. I was a young, very healthy and fit 19 year old and by the time I was done, my full B, average C cup chest was reduced to a normal B. They appeared a bit smaller and the pectoral muscles were rockin,lol.
Oftentimes breasts will shrink with weightloss, but if you are fit to begin with, that won't help.