One could be a story about a wooden 'cock' - like figure on a ship's mast (as in a weather vane in the shape of a rooster). Due to the weathering and decay over time, it's slowly shrinking. This could be part of a nautical adventure story where the sailors notice this strange phenomenon and try to figure out how to fix it or what it might mean for their journey.
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.
Imagine a story set in a magical garden. There is a statue of a rooster (cock) that is made of a special kind of clay. Every time it rains, a little bit of the clay washes away, causing the statue to slowly shrink. The garden's caretaker is trying to find a way to stop this shrinking and preserve the beautiful statue. This is a simple and family - friendly 'shrinking cock story'.