It's an actual documented medical condition, and I don't remember what it's called, but it's pretty much what you'd expect, "gigantomams" or something.
As with any other part of the human body, things can go wrong with the DNA programming, causing them to be underdeveloped or overdeveloped, but to be honest, having huge tits has got to be pretty far down the list of problems to have. It's okay, I'm sure someone will love you and your huge tits.