Yeah, exactly this.

As a girl I can tell you, OP, that when I tell my friends I slept with someone new their first question after "how was it?" is always "how big was he?"

I'm sorry because of course that sucks and society should change to be more genuinely body positive, but it's a fact of life right now. Your gf might have only been answering a direct question from her best friends at first, and once they know something like that they're going to keep bringing it up. It's okay to feel hurt, obviously, because we all want to think that our SO is showing respect to our feelings at all times. But I wouldn't say you should set fire to the whole relationship, because this is, unfortunately, a fairly normal way that girls talk. So if she's a good gf for you otherwise, I'd say to at least try to work on it with her or tell her your feelings and give her a chance to work on it. Without ever looking through her phone again.