The reason it has allot of power per litre is because it combusts 3 times in just 1 rotation, so you could say a single rotarie is the same as a 3 cilinder engine.
But where the piston engine is calculated at displacement for every combustion chamber, the rotarie is only calculated for 1 combustion chamber.
If you would see the displacement the same as with a piston engine, the RX7 would actually have a 3.9 litre engine.
In the time a piston engine completes it's cycle a rotarie completes 4 combustion. So you could say the 1.3 litre twin rotarie is compareble to a 2.6 litre 4 cilinder piston engine.