Question about the design of yoshi mufflers

Sorry if this is the wrong section, feel free to move it!

I just removed the ends off a stainless gen1 yoshi muffler, to find that the inlet into the muffler is 65mm, and the exit is 48mm.

Can someone please in detail tell me why this is and what relationship it has with the engine.

Or even a good few links to read would be good.

Cheers in advance

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