I'm guess it's an engineering decision based on what kind of hp it takes to generate the centrifigul force necessary and the what forces the mower can withstand when that motion comes to an ubrupt stop. The bigger the diameter the greater those forces become
The larger diameter riding mower makes good use of a belt and pully system to increase the rpms at the blade shaft, relative to the walk behind. When the blade hits something solid with this configuration, the belt slips or absorbs the energy rather than the crankshaft.
So a larger diameter direct drive walk behind mower may not stand up well unless it has a way to limit forces that can result from inertia or the sudden stop of same.
This large diameter walk behind mower would also become exponetially heavier as it becomes stronger and more complex requiring a drive drain. This now extra heavy self propelled mower has the capapbility of towing additional weight such as a person....and the stand-up or sit down sulky was born!
If you have ever used one of these you'll understand why they were replaced with the more effective
riders we see today.
So I guess the answer to the question is mower evolution?