The word "hypoplastic" means that something is underdeveloped or small in size, particularly in reference to an organ or tissue in the body. Full definition
Some women with hypoplastic breast tissue do find that even with good support though, they are unable to create enough milk for their babies and may need to supplement their milk. (breastfeeding-babies.com)
For mothers with hypoplastic breast tissue it also means your baby is stimulating your breast to make more milk. (breastfeeding-babies.com)
At our 6 week appointment, Angie informed me that I have something called Hypoplastic breasts. (birthwithoutfearblog.com)