Peanuts are calorie dense for a few straightforward reasons:

1. High Fat Content Fat provides 9 calories per gram — more than double carbs or protein (4 cal/g each). Peanuts are roughly 50% fat by weight, mostly monounsaturated and polyunsaturated fats. This alone accounts for the bulk of their calories.

2. High Protein Content At ~25% protein by weight, peanuts pack more calories from protein than most plant foods.

3. Low Water Content Fresh fruits and vegetables are largely water, which has zero calories and adds weight. Peanuts are dry — almost no water diluting their calorie content.

4. Compact Structure The energy stored in peanuts is the seed's fuel for germination. Seeds are biologically designed to store maximum energy in minimum space.

So it's primarily the fat + low moisture combination. A handful (~28g) delivers ~160 calories despite feeling like a small amount of food.