Do breasts always shrink when you lose weight?

Usually. Usually they do, but in some women the volume can be retained.
Most likely. 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.
Usually. Breasts are comprised of fat and glandular tissue. Some women have a large amount of glandular tissue with little fat and vice versa. Fat is lost with weight loss, which impacts the breast.
Yes. Since breast consist of fatty tissue and breast tissue, loosing weight will reduce the volume of fat throughout your body - including the fat in your breasts. Thus the breasts will get smaller with weight loss.