Gigantomastia also seems to be other breast tissue and not just fat. So if one looses weight and the breasts don't reduce, you could probably deduce its gigantomastia related (least I generally do).Breast tissue is mainly fat. If a woman puts on weight, as we all do as we get older, the breasts will get bigger.
Gigantomastia is usually very rapid, extreme growth over a short period of time, usually 3 months to 1 year. And the growth is continuous.