[DuSh]

Here's stock 35i muffler cut open (top picture) - you can see a lot of sound deadening material and a lot of open ended pipes - that makes sound waves to travel a longer route through insulating packaging and dissipate and bounce inside a lot before exiting muffler. Sound is muffled overall.

On a second photo you can see a MPE (it's for 135i, but it's same engine essentially) - you can see that there is no deadening packing or material and also pipes go straight through the muffler (there is just 1 section of perforated pipe to lessen a bit exhaust sound, but that's about it) - making sound exit almost unchanged. Hence louder "sportier" tone exhaust.