Penile dysmorphophobia or small penis syndrome is a disorder in which a man is convinced that his penis is inadequate. This is often attributed to a warped perception of the size of the penises of others, a belief that they are expected to have a large penis, and a preoccupation with the appearance of the penises they see in places like locker rooms.
More About Penile Dysmorphophobia
Often the perception of one's own penis size in comparison to the penises of others is wildly inaccurate because men see their own penises from a completely different angle than they see others. Penises viewed from the front look longer and more evenly proportioned. This warped perception, combined with societal pressure built around the idea that large penises are somehow better, can leave men feeling inadequate.