AITA for telling my DIL she eats much more than me so of course she is bigger

I’m so tired, my son and my DIL are staying with me to save money. I cook and we have a serve yourself policy. The thing is she will eat a lot more than me. For example I made chicken, I would eat only one chicken breast she would have three. Basically she eats a lot more than me and I don’t substitute for lower calorie stuff. I’m not going to use skim milk instead of cream for example.

She is overweight and has been going on about how it doesn’t make sense that I am much smaller. I asked if she was joking and she confirmed that she didn’t understand why she is gaining weight. I told her that she eats a lot more than me so of course she is bigger.

This started an argument about how I am shaming her and not being a good host. My son wants me to apologize but I find it ridiculous and this is common sense….