I want to tell the girl I like that I prefer other body types to hers
Do you have something against her body?
I don't like girls with huge bones
Even though it is the truth, it is not good to tell it to some people some times. It can hurt their feelings and people will think you are a bully for telling her how you feel about her body.
Why can't I do that? I'll do it anyways.
You are going to hurt her feelings and that is why I am telling you that you should not tell her that. There is no need to say something that straightforward, what good is it going to do for you?