Yeah, I agree, it's only in HBP that everyone starts talking about how Ginny is pretty, and that's when Harry falls in love with her. I would have preferred it if it said explicitly that Ginny had gotten D-cups, because noting and reacting to that would certainly have made Harry seem like a more normal teenage boy if nothing else, but as I said, the girl suddenly becoming hot, which seems more than likely to be what's going on here, is plausible enough. But it doesn't make for a very interesting story.