Can a hormonal imbalance cause one breast to increase in size?

Breast tissue: Well many women have breast asymmetry and with things like pregnancy they can have very different changes in breast size. However, I wouldn't make that assumption without having a breast exam done in case the change is from a mass in the breast or chest wall.

No: Since both breasts "see" the same hormones any effect would be on both breasts.

Breast: An increase in size of one breast is unlikely the result of hormone imbalance but you need to see your gynecologist for evaluation and management.