There's also confirmation bias. If you believe all skinny guys have big cocks (or all black guys have big cocks) you are going to notice much more the cases where this is true, and not so much the cases where it is not. Also, you are more likely to judge an average cock as being big just because it's on a skinny guy, than because it is objectively big.
Additionally, being skinny physically allows your cock to stand out more from the rest of the surrounding body parts, body fat and muscles, supporting even more your confirmation bias.
While a great stereotype to have, it also has negative consequences. If you are skinny and the stereotype is that you have a big cock, you will constantly question yourself if your cock is big and lives up to stereotype. And if it is objectively small, you'll feel inadequate because you don't live up to the stereotype. This leads to constant problems with body image and self esteem. Also, what's average in practice and real life is way smaller than what people think is average in their minds and the internet.
While it's a funny stereotype and topic to discuss, it's also damaging and dangerous. That's why they are called stereotypes and we should be extremely aware.
That's a totally on spot comment, finally somebody acknowledges it. The thing about "tall skinny guys" has definetely become a bit of a stereotype, and being 6'2 and 145 lbs has made me wonder at times if I'm a big exception, although from what I've seen in real life (lockers, showers, etc.), I don't think I am. I'm not even tiny, just about average, at least when hard, but it seems "expectations" are high and there are threads not just here, where almost every skinny guy that is posted has a huge cock (or at least it looks like it). Women and men commenting "oh, it's always the skinny guys", which irritates me a bit, but even if those guys that are posted, really are very big, I'd say those are the exceptions, and everything you mentioned about the confirmation bias applies.
Now obviously if you are overweight, some of your fat may really "hide" your dick and it may look an inch or two smaller than what it actually is, when measured bone pressed. But a guy, who is average body weight, won't magically gain 4 inches, if he loses some pounds, obviously, it just doesn't work like that. Neither will a skinny guy lose inches, if he gains a few pounds.
So us skinny guys are often fetishized and I guess it's normal that such stereotypes appear. Whether it's "the tall skinny guys" or the "the short skinny guys" (yes, some say you actually have to be short, lol), everything is exposed when you actually pull out the ruler and measuring tape and do a correct measurement and I have seen no evidence for correlation between body structure and dick size, apart from height maybe and it's still very very slight. For me personally, I haven't become "victim" of this stereotype yet, so no reason to be insecure about it and hope no fellow skinny dude becomes one, lol.
