I think it's pretty common that guys -- brother to brother, or sons with fathers -- talk rather frankly about their endowments, and that might include some comparison or revealing or jerking off together. Heh. Honestly, if I had my dad around to explain what this big dick thing would be all about, then I wouldn't have had nearly as many surprises. I suppose as long as all of this activity is consensual and that boundaries are well clarified, then there's no real damage to be done here.
While my brother was still alive, he found out that I was bigger downstairs and it made him jealous to no end. Hey, he'd been picking on me for years; it was long overdue that I had something to lord over him.