15Permanent: Different Cup Sizes
We've all got that one boob that is the slight overachiever. It's rounder, fuller, and possibly even half a cup size or more larger than its twin. This is normal, but it can become even more pronounced after women breastfeed.
Mom will usually notice while she is nursing that one breast
seems to fill with more milk or stand out a bit more than the other. This is especially true if the baby prefers one side to the other or if mom doesn't double pump. Some of this will correct itself after weaning, but since breasts stretch and grow to accommodate milk, a slight size change can still be obvious after weaning. In fact, it may be more noticeable if one breast ballooned up much more than the other while breastfeeding took place.
Breastfeeding Basics website points out that the breasts are not connected to each other and there is nothing to keep them perfectly matched when it comes to milk production or stretching. One breast stretching to accommodate more milk than the other may end in one side being bigger or heavier looking than the other.
Always nurse off of both sides and pump both breasts when pumping. This should help even out some of the difference in your favor when weaning occurs.