You get bigger if you eat more and you get smaller if you eat less. It's important to get all of fat, carbs, and protein for a balanced diet. Not saying that there isn't anything more to it, but for most people it really is this simple.

that's like asking "why is warren buffet rich?"

answer: because his money in was greater than his money out.

doesn't really explain much, does it?

specifically, calories in/out doesn't explain: satiety, insulin resistance, muscle anabolism/catabolism, or cravings.

there's much more to it then in/out. yes, thermodynamics applies. no, humans are a bit more complicated than a terrarium.

Nothing?

Okay, I challenge you to eat zero calories and still maintain weight.