It depends on how much of your breast is actual breast tissue, and how much is fat tissue. Mine are pretty dense and have been around the same size since middle school, through ups and downs of weight gain and loss, still a 38 DDD.
You will also lose fat from your back and sides, making your band size smaller, affecting the size of the cup, etc. etc. It's a crapshoot.
If I had to lose my girls to lose my belly, well, bye-bye girls, been nice knowing you. . .