You are looking at flow volume. A 50cc engine at 10,000 would need as much air as a 100cc at 5,000. It will be harder to charge the 50cc because of velocity. I have used a 25mm carb on a 100cc reed valve engine with good results. A good indication of what can be done with carburation is the current 125cc Motocross bikes, they are all reed valve. When you can get 35 HP from 125cc in stock trim, you are pushing the envelope. Metallurgy has been a large factor in the current high performance engines. Reed valves have one advantage over mechanical valves, they do not allow blowback. However, you have the restriction of the reeds and reed cage.

I recently figured the rim velocity of a 3" diameter crankshaft at 10,000 rpm. 10" X 10,000 = 100,000 inches per minute = 8,333 ft. per minute = 138 fps. This is enough to induce considerable inertia into the charge. The better two cycle engines have the transfer ports parallel to the crank and the intake and boost ports in the direction of rotation to use the circulation in the crankcase to assist the boost ports. The intake ports are directed to the rim of the flywheels to assist in charging the crankcase.

Two cycle design has definately changed since the introduction of the Schnuerle system.