Will weight loss cause a corresponding decrease in breast size?

Probably: As breast tissue is primarily made up of fat cells, general weight loss should result in reduction to breast size. That said, women's bodies are different in terms of where most new/added fat is deposited. It can be the belly, thighs, buttocks, arms, legs and breasts, so how much proportionately will come off the breasts with weight loss is hard to say exactly.

Possibly: When you lose weight you do not have control over what parts of your body lose over others. A healthy and diet exercise program will cause weight loss throughout your body. If your breasts shrink too much, talk to a plastic surgeon about your options for breast enhancement.