I am so confused

A characteristic of fats that makes them energy dense is their lack of water. Because they contain relavtively little water they can carry much more energy per molecule than any other kind of compound found in either plants or animals.

Fat contain twice as much energy as carbohydrates do, when they are used in respiration. This is because fats are compratively low in oxygen atoms (the carbon of lipids is more reduced than that of carbohydrates), so more of the oxygen in the respiration of fats comes from the athmosphere. In the oxydation of carbohydrates, more oxygen is present in the carbohydrate molecule itself. Fats and oils therefore a concentrated energy store.

Source: Allott, Andrew, and David Mindorff. Biology: IB Diploma Course Companion. Oxford: Oxford UP, 2010. Print.