I don't use a 21 unless the situation dictates it, NOT the customer. For instance, I have one backyard that my 36" will not fit through, so I use the 21". Small areas, I use the string trimmer.
I've got a couple of customers who have been jaded with the larger mowers that their previous guys used and tore up their yard. These customers don't want a large rider on their turf. Fine, the yards are not large and I walk it with the 36". I tell them I won't ride it, but I insist on the 36" Exmark and tell them I will cut it once with no obligation. If they aren't happy, they don't have to use my services.
So far, they are so pleased with the cut and look of their yard, I have the neighbors asking.
Now mind you, I charge a bit more for this. Once you get to walking fast behind a 36", you can get through a yard fairly quickly and a whole lot faster than with a 21".
If I had to put it into money, I figure the time difference for walking it. Let's say it takes me 30% longer to cut than with a rider, so their bill is 30% more. But on smaller properties, that is harder to figure out as some I woudn't use a ztr on anyway.
With a 21", I would charge them at least double if not more. But then again, I don't think I would do any yards entirely with a 21" unless it was a postage stamp size and I don't have any of these.
Good luck.