Why it is so offensive to call fat people fat these days?

I mean if you are fat, obivously I am gonna call you fat and not Victoria Secret Model. Yesterday I went to Zara with my friends to do shopping. Two of my friends tried the same bodycon dress in different colours. Both of them asked how it was looking and told me to give honest opinion. One of them is midsize and dress looked nice on her but another one is really overweight and she was literally looking 9 months pregnant in that dress so I told you are looking really fat in this dress and you should try something flowy and not bodycon. Now I could feel that she got offended. When we were eating I ordered diet coke because I avoid sugar due to acne so she taunted me by saying I will also have diet coke if I want to fit into that dress and it snapped me I told her yes you should. She has a very unhealthy lifestyle, no exercise, screwed sleep cycle and always eating junk food but will blame on PCOS and thyroid. She told me how it is offensive to call someone fat and I should be more sensitive and follow some body positive content on social media. Honestly I am really pissed on what I did wrong? EDIT : Just to clarify I did not gave unsolicited opinion, she literally asked how dress was looking and to give honest opinion then I told whatever I told. Also, I didn't mentioned anything about her lifestyle although personally I am against of it.