Here's a copy/paste from Boeings Jet Transport Performance Methods:
Did you notice that LHV decreases as fuel specific gravity increases? It might seem logical to assume that fuel that has a higher specific gravity would also produce more energy per unit of weight than a fuel having a lower specific gravity, and yet you see from the equation above that the opposite is true: lighter fuel has more energy per unit of weight.
This fact arises because denser fuels have a higher ratio of carbon atoms to hydrogen atoms. The energy of combustion of carbon is less than that of hydrogen. Denser fuel, having relatively less hydrogen, therefore produces less energy per unit of weight.
Interestingly, however, denser fuels have higher energy content per unit of volume. This is due to the fact that the higher density fuels, due to their molecular structure, can pack more molecules into a given volume such as a liter or a gallon. Although each molecule has slightly less energy, the number of molecules per unit of volume increases with density more than the energy content of each molecule decreases. The net effect is a greater amount of energy per unit of volume.
Where LHV stands for Lower Heating Value, expressed by Boeing in BTU.
Now my question is: do we wnt to know liters or kilograms ( what I thought and has been described in other posts before)?