Rotarys make high power for their size because of their volumetric efficiency, which is a measure of the volume of air going into the engine divided by the capacity of the engine. (for example a 1.0 liter engine at 100% volumetric efficiency intakes 1.0 liters of air per revolution). Obviously, more air going in means more fuel and thus more power.
Most stock piston engines only have a VE of 50-60%. A stock rotary is about 70%. Peripheral ported rotarys can go over 100% (which means that it is more air than the engine's volume). This phenomemon does not occur in normally aspirated piston engines (except in highly modified race engines). It does occur in turbocharged engines fairly often though.
On the other hand, rotaries use more fuel than a piston engine of the same size. This is due to their poor "thermal efficiency" (which means that more of the fuel is used to heat up the engine than the working gasses within it). This poor thermal efficiency is because of the very long and thin combustion chamber (more surface area thatn the same capacity piston engine).
All production rotarys made by Mazda use two spark plugs per rotor to ensure that as much of the mixture is ignited as possible. In reality the power output of the extra plug is marginal (5% perhaps), however it is mainly used to lower the emmissions from the engine.
However, the Le Man's R26B engine has a third "late trailing" spark plug even higher up the combustion chamber. This, combined with a variable length intake manifold and peripheral porting gives 700 JIS PS from what is effectively two normally aspirated 13B engines (350HP from a non turbo 13B is quite impressive).