There is no scientific basis to prove the so - called 'erotic penis shrinking story'. Such claims are often part of unfounded superstition or false rumors.
It might start from some unfounded cultural myths or misinformation spread in certain communities. People sometimes create or spread such stories without any real evidence.
I'm not entirely sure without more context. It could potentially be a fictional or anecdotal story about something that is shrinking in a metaphorical or perhaps a very strange physical sense. But it's a rather unusual phrase.
It is most likely not a well - known folktale. Folktales like Cinderella, Little Red Riding Hood are known worldwide. But this 'cock shrinking brother story' is not something that is commonly mentioned or part of the typical folktale repertoire. There could be some very local or family - specific story with this name, but not a well - known folktale in the general sense.
I can't really think of a traditional well - known story that uses this exact phrase in a non - inappropriate way. However, you could create a fictional story where a character named Cock (a rooster) is shrinking because of a magic spell gone wrong in a children - friendly fairy - tale - like setting.
I haven't heard of a particular 'cock shrinking brother story' before. It might be a very unique or regional tale. Maybe it's a story created within a specific community or family, and without more details, it's difficult to say what it is about.