If you include the weight of the oxidizer, it's a wash. Oxygen, with mass 16, is pretty heavy compared with carbon's 12 and Hydrogen's 1 when you have to combin

If you include the weight of the oxidizer, it's a wash. Oxygen, with mass 16, is pretty heavy compared with carbon's 12 and Hydrogen's 1 when you have to combine two oxygens with each carbon and one with each pair of hydrogens. The big win in "energy density" from normal hydrocarbons is that the oxygen is freely available from the atmosphere.