Question

Why do ashes from fire have a much lower mass than the wood that was burned

Answer 1

because when you burn some wood you don't just get ash, you also get energies like thermal energy(heat), light and also some gaseous compound are produced (like CO2, SO2 etc) for the combustion reaction(a reaction where something is burnt in presence of O2). So the energy reserved in the wood are transformed into heat, light, other compounds and some turn into ash.