Smaller tank size means higher pressure, resulting in rucher than expected? Hmmm, this is a good one. For most setups you use an exhaust fitting to pressurize the tank. This pressure would not vary based on tank size. Furthermore, you control the fuel mixture with the needle valve. If for some reason the smaller tank really was resulting in more fuel flowing to the engine you would just set the needle valve a bit more lean. The value of using the same size tank as you will use in the plane is to be able to measure run times.