How Likely is It for Breasts to Grow Back After a Reduction?

Unless you eat a ton of soy (has phytoestrogens) your diet is not likely to be a factor unless you gain weight. Existing breast tissue can become larger after a breast reduction even if you haven't gain weight. A few of the more common reasons (besides weight gain) are use of hormones like birth control pills (even changing types) or others at the time of menopause, endocrinological problems, further normal growth if the first procedure was done at an early age, pregnancy related, and in many cases unexplained reasons. A secondary breast reduction can be performed once the cause is understood. Without photos or an exam the exact technique best for you cannot be determined. Your risks should be about the same as before. 

Is there is no weight gain it is almost impossible for breast to go back after reduction. Weight gain after reduction may result in larger breasts if the body preferentially deposits fat in that area.

Increase in breast size is usually rare as long as a stable weight is maintained. Your age may put you at increased risks for changes in breast shape after reduction.  Bearing children will change breast shape in addition to changes in weight.

Dr. ES

If you are of adult size and are no longer growing ( which at 21 I am sure you are) it is unlikely your breasts will grow anymore. They will grow if you gain weight and may sag as you get older and/or have kids.

This is a good question concerning enlargement of the breast following breast reduction surgery.  This is a possibility if there is still some degree of hormonal stimulation on the remaining breast tissue.  It can happen but is not common.  On rare occasions we perform breast reductions on 15 or 16 year old girls who have very large breasts.  These are the women who are most at risk for regrowth.