The word "fat" is not a slur any more than the word "black" is. Sure, someone can use it with an intent to hurt, and if the only thing you know about a person is this single adjective you probably shouldn't be talking about them, but the word is just a description. And just like for "black", all the euphemisms offer nothing helpful and are largely spread by people who have not lived and understood the experience.
If you're worried about being fat-phobic the thing to be worried about is treating fat people like shit based on their physical appearance. Up to and including shaming them for "not putting in the effort" or lecturing them about how unhealthy you think they are based on the single point of evidence of their apparent weight.
And I have to say, I'd be WAY more fucking mad at someone calling me "rotund" then fat. Holy shit you have missed the mark on this.