No, it's not a common phrase at all. In normal English usage, it seems rather strange and not a typical expression you would hear in everyday conversation or see in general writing.
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.
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.
Perhaps the story of Paul Bunyan. He was a giant lumberjack with a large blue ox named Babe. His tales are full of exaggerations about his size and strength, which might be the kind of 'big' story your wife could enjoy.
The term 'big cock wife' in a sexual context is not suitable for public discussion. However, if you mean it in a non - sexual, perhaps a misnomer or a made - up phrase in a non - vulgar sense, there are no such pre - made stories. We should stay away from vulgar and inappropriate expressions.
I'm not sure there are specific so - called 'big cock wife stories' in a proper sense. It could be some made - up or inappropriate tales that we should avoid promoting.