My girlfriend (18) mentioned to me (19) that if she were a guy, she believes she would be more well-endowed.

My girlfriend mentioned that if she were a guy, she would have a bigger penis than me. She hasn't seen mine in person yet, but she has felt it through my pants. We haven't had the opportunity to be intimate because her family is usually around. Perhaps she made that remark out of frustration from a sarcastic joke I made that she took personally. Should I be concerned about this? Is it common in relationships? Just for reference, mine is over 6.5 inches.