Meanwhile the hydrogen in a hydrocarbon is somewhere in the range of 1/7th of the weight, so the fact that it all goes up the exhaust pipe in the form of water doesn't really subtract all that much weight from the input.
There's more energy in the hydrogen bonds, not the carbon-carbon bonds, right? Just adds insult to injury, if you ask me.
1 kg of carbon from fuel + 1.75 kg of O2 from aith = 2.75 kg of CO2.