I think the true limiting factor in RPM is the carb bore. It is possible to get a .21 with a 10mm carb whereas a .40 airplane engine is only at 7-8mm. The 10mm carb can let a lot more air through it than a smaller one allowing the engine to rev higher. If you ran a large carb with a high load the air speed through the venturi would be too slow to properly atomize the fuel. Equip a 40 engine with a large enough carb (14mm) and you now can have an engine that is happiest in the 25k range. Also the porting of the engine has a large factor in the equation.