This is a little long on explanation and will not use words like "pulse" or "reversion". Do NOT mean to insinuate any contributers are not sharp but as a professor once told his class--if I use words or examples some don't understand then both persons time has been wasted. The exhaust should be sized so the "ball" of exhaust traveling out the pipe will fit the inside of the pipe like the piston in an old time suction gun. This leaves an suction behind the ball that helps pull any unburned gasses out of the cylinder AND pulls a new fuel/air charge into the cylinder when the intake valve valve starts to open. It's called the "scavaging" effect. When the pipe is too large the ball fits too loose and has no suction. Of course, as usual, there are exceptions, such as a very high rpm engine where there are many more pulses(balls) so that even though there is less suction to each one there are so many more it results in a net gain. Also on super/turbo charged engines the incoming charge is under boost(pressure) so a very low restriction exhaust is desired. I can hear the guys with a background in fluid dynamics laughing their hineys off at this explanation but I hope it does show why proper sizing on the exhaust does have an effect.
