Big mufflers and air cleaners give more sound reduction with less flow restriction, hence more power per litre and less fuel consumption.
I'm not really clear on why the mower frames have stayed much the same size over the years, while the engine power has doubled and sometimes tripled. I can see why that happened with cars, we all gain advantages from increased performance at times (though less and less often, as roads improve). However it seems that deck cutting widths have not increased nearly as much as engine power has. Furthermore, some very inefficient transmission systems (notably the Greenfield type) have remained in use, wasting a substantial amount of the engine's output.
As you will have noticed, I regard the ride on mower industry as not having kept up with available technology over the years. It is only by increasing production volumes that improvements like hydrostatic transmissions will become both durable and affordable. The same thing happened with car automatic transmissions.
