[surferjoe2007]

The cause is unknown. It doesn't seem to be genetic. Women with the condition have normal hormones but their breasts are hypersensitive to those hormones. I think normally when a hormone hits a receptor there's a "done" switch which desensitizes to the hormone, and those exposed to a lot of hormones normally get desensitized even more. I'm betting they don't have that, so the same hormone keeps hitting the same receptor again and again.

Steroidal saponins, found in fenugreek for example, have a similar effect of unregulating receptors rather than having a direct hormonal effect. However there are also LH & FSH receptors which when unregulated increase hormone production too, so steroidal saponins can increase some hormones too. There is a rat study where a pregnant rat given a megadose of shatavari extract (also high in saponins) developed gigantomastia. Other rat studies without existing hormones such as pregnancy hormones did not show mammary growth from shatavari. Unlike humans non-pregnant rats don't normally have mammaries so there's nothing to unregulate.

I looked around and could not find any similar study from megadoses of estrogen. In fact I found this study saying that excessive estrogen does *not* lead to breast hypertrophy aka gigantomastia (nor does having an excessive number of estrogen receptors): http://www.ncbi.nlm.nih.gov/pubmed/2359804 .