I am that rail thin person who everybody thinks eats 4000 calories a day.
But everybody is wrong.
I work hard at maintaining my weight.
I count calories, and, yes, when I'm out to dinner with people I can and do eat large quantities of food, but those are large quantities I've made room for in my budget.
Science says that 60 year old good-life-living rail-thin man you're using as your "evidence" is most likely just like me. Unless you've spied on him for years you have absolutely no idea what his complete dietary habits are and your presumption that his weight comes to him easy is insulting.