I have looked at small engines. Once I saw Mitsubishi G350L (made around 1986), the displacement is 141cc, but it maximum power is only 3.5HP at 2200RPM. I saw another engine, Honda GX200, 196cc, 6.5HP at 3600RPM. And another amazing Bajaj (automotive maker from India) with its Qlue, the engine is 200cc but can produce 13.3HP at 5500RPM. Indeed they are different. But the difference looks to be too much.
My question is how come the engine power so different? What is the relation between the piston's displacement and its power? Those 3 engines are relatively close in the cylinder displacement.