I think I am going to refer to a girl from my gym as Ariana Grande.
Why would you call her that nickname?
She is just the girl who is least fat. I feel like it is only right for me to call her Ariana Grande.
While I'm sure that you calling her names may seem very light hearted, but you calling her Ariana Grande may be very hurtful for her and may make her feel insecure about going to the gym so I don't suggest that you do that. How about you make friends with her, and ask her what is a good nickname that she can be called with, instead of insulting her?
I only call her Ariana Grande because she wears the same barbie pink outfit every day.
You are better off just calling her by her real name. It is going to sound like you are making fun of this girl if you give her some sort of name like this.