Different Breast Implant Sizes for Slightly Asymmetrical Breasts?

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There are many different methods to manage breast asymmetry including: different implants or fill, lifts, reductions, etc.
The approach you suggest is reasonable. Be prepared that it may take more than one operation to achieve the best possible result.

Volume is only one factor to consider when trying to determine the optimal correction for breast asymmetry. The anatomy of your breasts, as well as implant dimensions, specifically height, width and projection also play a major role in bringing about symmetry.
For example, a low profile implant, for a given volume, may be more than 2 cm wider than the same volume in a high profile implant. To compensate for that, the high profile implant ends up proving more projection and more upper pole projection. Even though the volume may be the same, the breasts will look very, very different. Even for the same style of implant, in some cases, a small volume disparity looks better than a different sized implant, because of the disparity in implant dimensions. The amount of asymmetry will help you and your plastic surgeon make the best decision for you.
You would really benefit from an evaluation by a board-certified plastic surgeon. Best of luck!