You would sacrifice some low-end (RPM) performance for high-end performance. A larger exhaust offers too little or no resistance at the low-end, forfeiting the Bernoulli effect (resistance = velocity, cf. a garden hose) and the Venturi effect at the intake; however, with more exhaust moving at the high-end, you gain those effects back without the overbearing restrictiveness of a smaller pipe.
It is an either-or situation, and you choose based on your application, same concept as a 4-to-1 header versus a 4-2-2 (tri-Y) header.
Now, if someone could make an adaptable variable width exhaust...ka-ching!
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