I read this on Big Bust Support:
"If exercise doesn't increase the bust, will it work to decrease it?
The answer is yes, and no.
Remember that breasts, for the most part, are made up of fatty tissue. Exercising can lead to fat loss, which in turn leads to smaller boobs.
Exercise can also tone up the pectoral muscle, helping to lift and define the breasts. I discuss the myths of breast reduction exercises on my Breast Reduction Exercises Page.
There are many exercises that focus on strengthening the chest wall, incorporate them into your workout routine for the best results. I list and explain these exercises on my BigBustSupport Exercise Page
Before exercising,remember to find a good sports bra. I talk about the best sports bras for large breasts here."
The page says weight loss deifnitely helps, which seems quite logical.