AITA for saying this to a colleague

I work in a busy cafe in the UK and have a colleague who I am very friendly with. To protect her identity let's call her Sophie. As it is a very busy establishment there are a lot of jokes flying around to keep everybody's spirits up. Now Sophie is concerned and thinks that she is overweight. One day she was saying something along those lines and I said 'nah you're just a Chunky monkey.' I meant this to lighthearted and a cute name, she however did not see it this way and was upset and now I feel bad. AITA?

Edit: yeah I Was thinking along these lines. Let me just say at least that we are still good friends and laugh about it now and we've posted this together