> The energy of our most violent explosive--nitroglycerine--is less than 1,500 calories per gramme.

Well, part of it is that you don't really want "violent explosives", you want a controlled combustion. Once you take that limitation out of the way, turns out that explosives don't have very high energy densities, they are actually, for the most part, less energy-dense than food. Take a look at this table: http://physics.info/energy-chemical/

(Funnily enough, was discussing this over lunch just now).