Breasts are made of a network of ducts, covered by a layer of fatty tissue. During pregnancy, estrogen and progesterone enlarge the milk ducts and multiply the glandular tissue that produces milk. After birth, estrogen and progesterone drop and prolactin and oxytocin rise. Prolactin makes milk production and oxytocin releases it into the ducts. Extra blood and fluid fill the breast just after birth to supoort your body adding hormone receptors in the breast to make milk. The blood and fluid surrounds the ducts and this extra pressure is what makes your breasts feel full between feeding. This blood and fluid reabsorb around 6-8 weeks once supply is established and you won’t feel that full/soft feeling except when you go a really long time between feeding or pumping.
Breasts go back to prepregnancy size when supply regulates around 11-14 weeks but continue to make milk. When you wean from breastfeeding, it can take several months for prolactin levels to return to baseline (which is why you may still see milk for months after weaning).
Once you stop breastfeeding, the milk making structures actually self-destruct – a process that involves massive cellular suicide, and the removal of the debris. Around 6 months after weaning, the milk-producing tissue is replaced with fatty tissue. If you return to your pre-pregnancy weight, your breasts most likely will return to the same size.
They may not be as “perky” because the skin is a bit more stretched and the connective and fatty tissues in the breasts often shifts during pregnancy and breastfeeding. While they may look smaller after weaning most of us can expect that our breasts will return to a similar size as they were pre-pregnancy. They’re just a little more lived in and well loved.