Why are my breasts growing bigger on implanon?

Unbalanced hormones: Implanon is progestin only and that can unbalance hormones. Estrogen is a growth hormone and too much can make all of you "grow", and even too much synthetic progestin can block your own natural Progesterone function and cause weight gain. Unfortunately breasts and then waists get weight storage first it seems. This is a common side effect and should have been explained before implantation.

Hormones: Nearly any oral or implanted contraceptive has intended consequences of hormone imbalances that result in the inability to conceive but may have unintended or desireable/undersireable side effects of weight gain, breast enlargement, acne, hair growth, blood clotting, etc.