Why would a woman naturally have large features?

I know someone who is the same stature as I am (5'2, and 100 something lbs). She and her elderly mother both have extremely large um, "chest". Her sisters are not this large and are flat chested. I am wondering if there is a genetic anomaly or a hormonal imbalance that causes this. I've seen it on people before, where one relative has the issue and the others don't. And my own relatives have larger (you know what) than I do. I hate it, and wonder if its normal.