An interesting picture as one of the women is genetically XX and the other XY.
Breast Development in the Genetic Woman
At birth the rudiments of the functional mammary gland are in place for both boys and girls: the nipple and areola are formed along with a rudimentary system of mammary ducts extending into a small fat pad on the chest wall. The mammary gland remains a rudimentary system of small ducts until puberty when in a genetic woman the advent of estrogen secretion by the ovaries brings about the first stage of the four stages of mammary development: mammogenesis. Only the breasts of child bearing women go through the next two stages - lactogenesis and lactation. Finally, the breasts of all post-menopausal women go through involution, although hormone replacement therapy can affect this.
Mammogenesis commences at puberty with the onset of estrogen secretion by the ovaries, usually between the ages of 10 and 12 in a genetic girl. Estrogen (often spelt 'Estrogen' in American English) stimulates breast growth by acting causing enlargement of the mammary fat pad, one of the most estrogen-sensitive tissues in the human body, as well as lengthening and branching of the mammary ducts. The development occurs according to well-defined milestones called Tanner stages:
10 - 12 years
Increased nipple size
+ 6 months
Increased areola as well. Breast bud (small tender lump behind the nipple)
+ 18 months
Nipple – areola complex increases. Breast size increases
+ 24 months
The areola is a separate mound above the breast
+ 42 months
The areola becomes confluent with the breast
leaving only the nipple proud
Breast development, Tanner Stages I to V
The levels of estrogen required to cause breast development are surprisingly low - until stage IV, the growth of the breast in a girl takes place with estrogen levels similar to an adult male. That is why about 40% of male children also initiate "Tanner I" type mammary development during their puberty due to the tendency of their testis to secrete significant quantities of estrogen's in early phases of its development. However, as testosterone secretion increases the breast development ceases and very few boys reach the Tanner II stage.