Milton,

We have to consider that the exhausted gasses get cooled by contact with the relatively big area of the sheet metal skin of the muffler.
Then, the volume of the gasses is reduced between inlet and outlet.

As you know, the main purpose of the mufflers is to quiet the noise by interference of the sound waves, and get the back pressure from friction of the gas and changes in speed and direction.
Normally, the resonance and overcharge that happens in the pipes don't happen inside mufflers.

The manufacturers try different shapes, chambers and diameters, until they get a compromise among noise reduction, backpressure, available room and production costs.