The loss in "performance" was due to the lower pitch. The pitch gives you the theoretical distance that the prop will pull the plane forward with every revolution. (Friction, drag, etc. reduce the actuals "speed".)

But speaking theoretically, assuming that your engines turn at 10,000 RPM, the 9 X 7 props would pull the plane at a top speed of about 66 mph, but the 10 X 4's would only get you about 38 mph. Quite a difference! []
Dennis-