The female breast is composed primarily of fat and connective tissue plus lobules and ducts for milk production and delivery. The pectoralis muscles form a thin layer beneath the breast, as seen in the figure in the link. An athletic female can enlarge the size of the pectoralis muscle and the laterals muscles, just as male athletes can enlarge their "pecs" and "lats".
Exercise burns fat so athletic women often experience reduction in breast size. Of course, other factors such as diet and genetics also influence the effects of exercise on fat burning.
So, athletic females with enlarged lateralis muscles and pectoralis muscles may have a larger overall bust size but the cup size of their breasts might actually be reduced.