About Breast Tissue. Fat tissue often makes up the majority of tissue in the breasts; the fat cells expand as you gain weight. People with larger chests have more fat tissue, not more duct or lobule volume, which is the lean tissue required for breastfeeding. What is fat necrosis of the breast and what are the causes?. Rob Gage/Getty Images Fat necrosis is a benign breast condition that can occur in your breast. It consists of fatty tissue that has been bruised, injured, or has died. Natural decline of estrogen. One of the main causes of aging changes in the breasts is a natural decline of the female reproductive hormone estrogen.