Have you looked into doing "ab vacuum" exercises? I would prefer to have strong abs over good looking ones any day.
As for bigger boobs, estrogen helps them grow. It is either by water retention or fat storage that makes breasts bigger from what I know. Fat cells do not die, but inhabit our bodies until removed (liposuction) or death. Fat cells generally grow bigger or smaller, depending on influence on the cell.
From what I've read, certain "types" of estrogen have an effect on a specific body area when elevated. Some women get stomach fat, hip fat, or thigh fat. I'm not sure about the breast however. I hope this gives you clues as to what you may do to return your breasts to a bigger size.
If I were you, I would simply avoid working out the chest much and focus on the abs by exercises such as the ab vacuum. As long as you're burning fat at the abs by the muscles requiring energy there, then fat storage will eventually occur again in the breast given adequate dietary intake. The reason I brought up various forms of estrogen should start to make sense as "estrogen" refers to a generic term of estrogens to prevent common folk confusion. The differing forms of estrogen guide fat storage. Again, I don't know the details of which estrogen affects what area.
My girl friend's breasts became a cup bigger on birth control, even though I am opposed to the practice of oral intake of synthetic estrogens.