Here's why:

* Fatty acids are highly reduced molecules, meaning they have a lot of carbon-hydrogen bonds. These bonds store a significant amount of energy that can be released through oxidation (burning).

* Carbohydrates are less reduced than fatty acids, with a higher proportion of oxygen atoms. While they still contain energy, they have fewer carbon-hydrogen bonds and therefore store less energy per gram.

* Carbon dioxide is a fully oxidized molecule, meaning it has no more energy to release through further oxidation.

In summary:

* Fatty acids > Carbohydrates > Carbon dioxide in terms of potential energy content.

This is why fats are a more efficient energy storage form than carbohydrates.