I have known naturally thin people who can easily eat double what I do. And, no I do not try to out eat them! In fact, I do not eat at a buffet because I would not eat enough to justify the cost.
I am not sure why you have such issues with those who are overweight but, have it. Being so judgemental says more about you than my weight says about me.
Then you wouldn't have to worry about paying the buffet premium.