Eh, you can imagine it going either way; maybe someone who’s worried about it is more likely to sign up for a dick measuring study?

There’s so much psychological baggage here that I’d be really suspicious of any stats. The Kinsey Report famously found that gay men were almost half an inch bigger than straight men, and for a while that lead to a lot of speculation around sexuality being driven by maternal hormonal differences, but I think these days it’s pretty much accepted that it was probably just selection bias (that bit of the study was optional).