As breasts are filled with adipose, fat tissue, weight loss means fat loss in the first place, then losing fat is the worst process for your breast size. In order to take previous state back or to improve a general composition of your breasts we need to strengthen muscles around them AND to add more fat, to target the breast area. Sounds like impossible?
If they are too heavy, breasts will sag. That is the reason why slow losing of weight is recommended especially for woman. You must follow precision weight loss program. That will give time to your skin to strengthen naturally as it tends to do so. Strengthening pectoral muscles are of crucial importance to raise your breasts up, much more if they are heavy. Those muscles can fill up the excess of the stretched skin. Don’t avoid chest exercise with weights each day or at least twice per a week.