Living organisms are regulated by feedback paths. Paths, plural. Many feedback systems. When you adjust a parameter (e.g. CI) a great many other parameters adjust in the background according to the organism's inborn feedback controls. These controls evolved, which means their one criteria is successful reproduction in whatever conditions existed when they evolved. They don't have to be good or make sense, they just had to increase the odds of successful reproduction at some point.
That makes for an insanely complicated set of relationships. People who have actually worked with feedback systems (engineers, some scientists, a few others) understand just how complicated the behavior of a single feedback system can be. Layered feedback is exponentially more complex. Most people are just flat-out ignorant and will catastrophically underestimate the complexity...as demonstrated by some posters on this thread.
And, in most cases, these "skinny people" are simply not eating as much as you think - unless you follow them all day, every day for a week/month and log their intuitive caloric intake. It is rare to find people with such sky-rocketing high metabolisms.
My son has always been a good example of the skinny kid that appears to eat anything he wants. He has always been athletic, playing soccer and other sports. If you saw him at dinner time at home or after a game, he would be eating an entire Jumbo Jack Combo, plus dessert. That's a huge bacon cheeseburger, giant curly fries, large soda and ice cream or whatever he decided. Besides that, he probably only had a bowl of cereal that day. Total calories was still pretty low based on his activity level, thus he remains skinny. He never really at much until dinner time, so it was basically a form of intermittent fasting.
On the other hand, my best friend while growing up also ate "whatever he wanted". He started the day with chocolate milk, donuts and bacon and eggs. Ate fast food for lunch and dinner, snacked and drank sodas in between and was morbidly obese by his teenage years. He also played football, but all those calories outweighed his activity.