Immunity is passed from mother to baby through the milk. Breast-fed babies are also thought less likely to become overweight. There is some speculation that breast- feeding may also protect against heart disease in later life. It is also theorized that babies who have non-human milk early in life are more liable to develop allergies. Nursing also stimulates release of a hormone from the hypothalamus in the brain, which aids the shrinking of the uterus back to normal size following childbirth.
Breast size is immaterial to successful feeding.
Even quite flat-chested women may manage to feed twins. During pregnancy, the glandular tissue of the breast proliferates in readiness for lactation. When the baby suckles, the hormone oxytocin is released from the pituitary and stimulates milk production. A continued flow of milk depends upon the demand made by the baby. Most women who want to feed successfully can do so.