There are quite a few long and informative threads on this, Do a search so the people who donate their time to this forum don't waste it by continually duplicating previous work

Regards
pat

I agree with Pat about in-depth searches -- but, in brief:

Thermodynamically, the power from a given mass of fuel is related to its phase (e.g., gas vs liquid) and to the energy release from oxidizing reactions (breaking of chemical bonds in the fuel, and making of bonds between fuel elements and oxygen).  Carbon produces much more energy for a given mass than does hydrogen -- therefore the lighter elements, which have less carbon, have a lower 'heat content' for a given volume.  You can do a simple stoichiometric calculation for a particular fuel molecule, or for a known mix of molecules, get the heat energy release per mole or gram, and this is a first-level answer to your question: lighter fuels have lower effective energy per gram, and liquid fuels are denser than gaseous ones under sane operating conditions...  

There are other considerations, of course.  Gaseous fuels are often easier to carburete (mix with oxygen), and can often be combusted with fewer by-products (pollution) under engine service conditions.  In many cases -- for example, operating a forklift inside a warehouse -- these kind of advantages outweigh the increased fuel volume, special containers, higher gph consumption for equivalent output hp/torque, etc. that the gaseous or light fuel use entails.

hduncan, you have lots of reading and research to do to get a detailed story.  But if this was only a casual question, this might be enough.