Fuel, to a ped, is a mixture of gas and oil. The main jet regulates the flow of gas into the air stream passing through the carburetor at full throttle, but it is only one factor. Some others are the level of fuel in the float bowl, the pressure differential between the float bowl and the carb venturi and the design of the emulsifier (how much air is mixed with the fuel in the emultion tube).
I don't know about Puchs specifically, but all of the above and I'm sure, some other factors can change the required main jet size.
Hope this helps,