Tell her, that it's for 17 year olds to lay their head on when they sleep over with you... start to walk away and say "see you in 7-8 years." then look at her parent with a grin...
Tell her it happened before you learned what PCT was?
You could always tell her that it's not polite to comment on other people's physical differences. And mumble under your breath about parent's not teaching their kids any damn manners anymore.
Or you could reply that you work really hard on your physique and launch into a discussion on muscle function and growth. After about a minute her eyes will glaze over when she realizes she's LEARNING and she'll interrupt you with a "oh" and walk off.
I got asked this a couple of days ago by the 10 year old daughter of a co-worker of mine... lol, what are you supposed to answere to something like that?