It depends on the population density. You may have a perfectly well designed route for the area, but there are only so many people per hour that want to take a trip. You can delete routes and make people walk further, but that makes the trip take longer and not everyone can or wants to walk a long ways to the bus stop.

Different population densities have different optimal vehicle sizes. It's the same reason a small city airport might have one or two regional jets per day serving it instead of 2 747s per week.

Yep, you definitely want a range of bus sizes. Some areas are served perfectly well with a couple of these

https://en.wikipedia.org/wiki/Thomas_Minotour