Why are you girls afraid of someone calling you "thick"?

My wife calls her self fat all the time and when I call her "thicc" she gets mad at me. I don't understand what's wrong with calling a girl thicc. I know that I called her fat but isn't "thicc" just a nice way of calling a girl fat? Isn't it just a way to get her to see that she is fat in "a good way"? I guess I just don't understand how people say "thicc" is a bad thing when its clearly a compliment.