I'm gonna make fun of my friend because she has small boobs.
That's terrible! Why would you make fun of your friend?
She looks silly without any breasts.
People are very sensitive about their body shape . It's rude to make fun of someone's appearance. They will resent you for it.
I didn't say I was going to make the insensitive quip to her face. I'm going to say it behind her back so it's not really rude.
Even if you're not going to make fun of her to her face, it's still wrong. Insulting her behind her back is cruel and will potentially cause other people to be mean to her as well. She is your friend and you should be nice to her instead of doing things that will make her life difficult.