Q&A: One Breast Is Fuller Than The Other, Now What?
This just goes to show that a baby who is nursing well can drain your breast more effectively than any pump. Your body probably started making more milk on the one side to accommodate baby's single-sided nursing. Yes, your boobs should naturally even out once baby is back to nursing on both sides, but you can help speed things along by letting baby start feedings on the smaller breast for a few days. Also, after feedings, you can pump the smaller breast with a good quality pump to stimulate more milk production. (Don't worry if no milk comes out — it's the stimulation that actually helps.)