Well, beasts are partly fat, yes, but they are also glandular tissue which develops during puberty in the presence of estradiol. Unfortunately, not everyone has the same potential for breast glandular tissue development. Much of that potential is determined by genetics. Part of it is determined by the presence of estrogen receptors in the epithelial cells beneath the areolas. Also, overly high levels of estrogen is thought to stunt breast growth. Studies have revealed that many trans women on HRT maintain high levels of testosterone. The presence of testosterone (DHT) will inhibit breast tissue growth by blocking the effect of estrogen. Breast tissue development in adolescent girls proceeds over five or more years, then it stops. Very little, if any, breast tissue development occurs in women once puberty is complete until pregnancy causes the swelling of milk ducts. Personally, most of my breast growth occurred on weak phyto estrogens. When I switched to bio-identical estradiol, growth slowed dramatically. I then opted for implants to reach my desired size. Most trans women seek breast augmentation to make up for disappointing breast development.