Explosive substances not only have a lot of internal, chemical energy, but they also start easily and will produce lots of gaseous products.
The reactants (the explosives) are solid or liquid, so during the reaction their is an enormous increase of volume in very short time + lots of energy coming out.

TNT = trinitro toluene forms the products: NO2, CO2 and H2O. All gaseous products.
During its explosion, only one gramme of TNT can cause, in a splitsecond, several liters of gas.