Does anyone know if that's true? I've been reading that BC how that increases some temporary volume by increasing estrogen but it also contains progesterone to counteract that and to prevent uterine cancer. And how fennel which contains that increase breast tissue in rats, but also in their uterine.

I also found this on BO:
Bovine Ovary Extract - There are no published studies of the safety or effectiveness of bovine ovary extract in humans. Unlike the other herbs discussed here, websites selling the product say that bovine ovary extract stimulates the pituitary gland resulting in an increase in prolactin and growth hormone levels.

If this is true, there are potential risks. Prolactin is a hormone that rises almost ten-fold during pregnancy to stimulate breast development and milk formation. High levels of prolactin can cause infertility, irregular menstruation, reduced libido, and possibly increase the risk of stroke.

Growth hormone is needed to stimulate growth in children. High levels of growth hormone in adults who don't need it can cause the development of diabetes, abnormal growth of bones and internal organs such as the heart, kidneys, and liver, high blood pressure, and atherosclerosis (hardening of the arteries).