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The thing with exhausts is to eliminate backpressure while increasing the velocity of exiting exhaust gases. Think of a hose with an adjustable nozzle on the end that can make the opening small or big. Secondly, assume the water pressure is constant. If the nozzle's too small, not enough water comes out, but it exits at a high speed. Too big and a lot of water comes out very slowly ... too slow, in fact.
That's the way an exhaust (the "hose") works except the exhaust gases (the "water") exit in pulses -- four pulses for four cylinders, eight for eight cylinders, and so on. The idea with an exhaust is to match the piping size to make these pulses leave the muffler as quickly as possible but without restriction.
In essence, you don't want backpressure; you want extremely high exiting velocity for exhaust gases.
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