The breast is made up of three types of tissue: glandular, fibrous, and fatty (adipose) tissue. After puberty, the amount of glandular and fibrous tissue will remain unchanged in an adult, non-menopausal and non-pregnant woman regardless of bodyweight.
As the body loses fat from the entire body during weight loss, this will result in smaller breasts. At the same time, fat will also be lost on your waist, hips, shoulders, and more, your proportions will remain relatively the same. The aesthetic result of your breasts may even be improved as their size diminishes.