[caldrail]

I'm afraid it's firepower. MG's fire small lumps of lead, whilst cannons fire larger lumps of lead with explosive tips. I guess you could figure out why cannons are more effective. This is why conventional wisdom at the start of WW2 suggested that a pilot should get as close as possible before firing. Hitting the aeroplane might not help you. You need to hit the pilot or his engine, perhaps his fuel tanks, or something equally vulnerable.

Galland used to say "You should smell your enemy" - meaning the burned castor oil from a WW1 engine of the plane in front of you.