In patients with greater degrees of asymmetry, the breast "envelope" is tighter on the smaller breast and looser on the larger one. The nipple on the larger breast is lower and the areola is usually larger, because the nipple and areola are part of the same expanded ectodermal appendage that forms the breast. We need to use a larger implant on the smaller side to help correct the size discrepancy. Ideally, we would like to take up the slack more on the larger side, which tends to be saggier, but we have to use a smaller implant (so this side is not too big) on the larger side. A small implant does not take up much slack in the breast tissue, so the nipple will remain lower on the larger side. This explains why a breast augmentation will not correct significant asymmetry in which one breast is significantly larger and saggier than the other. In this situation, a simultaneous breast lift is usually the better option. This way, the breast envelope can be tightened as necessary. The nipples can be more symmetrically placed at the same level and the areolae can be reduced to provide a better match.
In most cases, however, a mild degree of asymmetry does not bother women enough to have the additional scars that come with a breast lift. It is unusual for me to perform a breast lift on just one side. Usually, it is better to do it on both breasts. The breasts and nipples appear more symmetrical this way, because a lifted nipple looks different from a natural one. The border of the areola, which has been incised with a scalpel, is more distinct than a natural areolar border. It does not fade away into the surrounding skin like a natural areola. Better to have matching areolae. Additionally, many women, whose nipple is low enough on one side to justify a breast lift, benefit from having both breasts lifted anyway because even the smaller breast is a little saggy.