Mostly the volume of food.
Your stomach has mechoreceptors (sense stretch) which gives you the “I’m full/stuffed” feeling. Gastric bypass surgery works by reducing the volume of your stomach so less food makes you feel more full. However, this is different from not feeling hungry. Many people will eat even when they’re not hungry just because they don’t feel full, this is one of the many factors leading to obesity.
Ghrelin is one of the hormones that make you feel hunger, though a lack of ghrelin doesn’t make you feel full it just means you aren’t actively hungry.
Calories are a measurement of the energy produced by burning the food we eat, they aren’t units of mass.
The other measurable parts of food, how many grams of this or that (carbs, fats, protein), allows us to measure what the caloric equivalent would be. But the body never knows anything about the idea of Calories.
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