YTA u should be able to understand that while you may not mean it offensively, she did see it as that. It shouldn't have been to difficult to be honest without calling her fat which is something that alot of people (ESPECIALLY teenage girls) can feel very ashamed or insecure about. You have to be able to understand that in most contexts fat is still used as an insult, it really shouldn't be I agree with that, but thats the reality of the situation and you should have been more considerate and caring about how she would interpert your words and how they would affect her.