The female breast tissue is combination of glandular (milk producing glands) tissue and Stromal (supporting and fat) tissue. During pregnancy, glands in the breast produce milk and it is transported through the ductal network to the nipples. These glands and ducts are supported by the fatty and fibrous connective tissue, which collectively give the breast its shape and size.
Majority of the female breast size is due to the presence of the fatty tissue. The amount of fat present differs in each breast (size asymmetry between right and left) and among different women. The size of the breast glands and fat tissue changes with each pregnancy and breast-feeding, with weight gain and loss and as the female ages.