Also remember that an oversized engine will put a greater structural load on the plane. The landing gear and frame around it will experience much greater loads with a heavier engine. Same applies for the firewall. You could beef up the structure to take the heavier stresses, but then you would add weight and negate some or all the power gained by oversizing the engine. Also remember that gassers induce greater vibration stresses on the airframe than glow engines (or especially electrics) do.