The twins aren’t identical.
It’s totally normal for breasts to look slightly different from each other—it’s called asymmetry. Breast asymmetry is defined as a difference in form, position, or volume of the breast, and it affects more than half of all women. In fact, your entire body is asymmetrical; one side of your body is completely different from the other (although it’s not usually obvious to the human eye). During your menstrual cycle, your breast tissue and size can change too. They may feel fuller and more sensitive when you’re ovulating, and get bigger from water retention and blood flow.
Generally, slight differences in a woman’s breast are not a concern. Some key differences, though, could signal a problem. If differences are greater than a cup size, it may cause psychological distress. Occasionally, an underlying condition, like fibroids or scoliosis, may cause asymmetrical breasts. Some differences—like nipple retraction, or breast swelling or dimpling—may be a sign of breast cancer, particularly if they appear out of the blue.