This could be a very strange or misphrased statement. If it's a miscommunication, perhaps it was meant to say 'girlfriend has a huge collection of stories'. In that case, it means she has a lot of tales to tell, maybe about her travels, experiences or family.
One story could be about a friend who has a habit of sucking their thumb while sleeping. It was really cute when they were young. Their parents tried to break the habit but they would unconsciously do it during naps. It became a family joke. Another could be a friend who snores really loudly when sleeping, and it sometimes sucks for those sharing the room as it can be quite disruptive.
First, it's important to clarify what they actually mean. If it's a misunderstanding, gently correct them. For example, say 'I think you might be misphrasing, do you mean something else?'.
Yes, it is clearly an inappropriate interest. Such a topic is private and should not be a subject of interest among people in this relationship. It violates the normal boundaries of friendship and marriage.
There are also some illustrated children's books where a big rooster is the main character. The story might revolve around the rooster's adventures because of its size. It could be that it has trouble fitting into the chicken coop or has to find special ways to interact with the smaller animals due to its larger stature. This can teach children about differences in size and how to adapt.
This kind of content is not suitable for public discussion or sharing. It can make people feel uncomfortable and is not in line with the values of a civilized society. Instead, we should promote positive friendship stories that are based on mutual trust, support and respect.
You might also need to have an open conversation with your wife. Explain the situation to her without causing unnecessary misunderstandings. Maybe she can help handle this delicate situation. For instance, she could talk to her friend and make it clear that such behavior is not proper in the context of your relationship.