I don't buy that manufacturer's argument; for me, same diameter in and out is more than sufficient.

As I explained above, the exhausted gasses get cooled inside the muffler and the volume decreases; hence, the speed (and friction loss) in the outlet will be smaller than in the intake of your particular muffler (unless it is a dual muffler with two inlets and one outlet).
Besides, the guts of the muffler make a difference in counterpressure and proper evacuation of gases that that manufacturer does not mention.

No much to do about an over-concerned costumer.

In general, two stroke engines with excessive exhaust restriction loos power and work hotter.