Why would he nickname me this?

I met a guy friend of a friend of mine. He likes me and we talked on the phone just two but we've never seen each other in person. I don't know how tall he is or what his body is like. he's only seen a picture of me from chest up. According to my friend, she says he is constantly asking her about me and that he has nicknamed me "big booty."

Why would he nickname me this?
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