Ok, how does that explain why some people want to eat even when they're full, while others feel full after eating a grape (exaggerating)?

It doesn't.

Weight is a direct results of the calorie equation.

Appetite is regulated by a bunch of different factors. Actually, when you think about it, someone could live just fine being constantly hungry (so completely broken appetite signals) in a food scarce environment.

You’re fighting a billion years of evolution screaming at you to bulk up in case there’s a famine. Good luck.

That's a bit illogical isn't it? Better to put the food in your pockets than in your stomach.