Well not to me, that's an assumption. I'm trying to explain the social component not the individual one.
I mean your entire comment was about rebelling against the notion of some desirability. That inherently suggests that size and attitude have some social link because you use it as a motivating factor to employ a fuck it attitude.
Again I'm not saying that everyone who is larger has confidence...blanket statements are by and large false and conceal a more nuanced truth. I'm say society values being larger over being average it smaller, that system of value can do a lot to Influence ones confidence and demeanor. It's a matter of who is getting positive feedback and reinforcement from society that's all.
How do you get positive feedback and reinforcement for a physical aspect that is, for the most part, an unknown? Most people see very few penises in the course of a day. You may have seen some in the locker room, all (usually) flaccid. There's no way to know who s packing a huge erection based on that. And other than that, and some media filler piece about some rumor and innuendo about a celeb (likely started by his publicist), we pretty much know nothing about the cock size of 99.9% of the population.
So how is this supposed positive feedback generated? I have a quite large penis. I'm even a nudist, so more people see me, and I see more guys. I don't go around complimenting their junk, nobody gives me "feedback", It certainly doesn't happen in public, either. Sounds like fantasy, but I'll entertain a reasonable answer.