I’m guessing it’s based on the outdated notion (especially if you interpret it as NBP, like I used to do) that 15cm is the average. (And therefore anything below that = “small”, I guess..) Sounds like a pretty dumb thing to get wide circulation at any rate.
Wasn’t there a hit in Germany some years ago called “20 zentimeter”? I’m assuming it had some similar message, and that you had the same situation then, with every guy pretending to find it funny and not be bothered by it.