> The ideal size for a bus goes down

No, it doesn't, because if you have lots of small buses then high-traffic areas will get clogged with lots of little buses loading and unloading and just physically taking up space in bus lanes. If anything, you generally want _bigger_ buses; most large cities size buses as big as the road infra will take (with the exception that a lot of cities won't consider double-deckers even where they'd make sense, due to lack of familiarity). Now, some of that is drivers, but some of it is just capacity.

(Or sometimes slightly bigger than the road infrastructure will take. About once a decade, Dublin Bus tries articulated buses, which then get stuck on roundabouts, reminding Dublin Bus why it doesn't have articulated buses.)

The mass difference between a big bus and a small bus also isn't generally _that_ huge. A smallish single-decker is about 10 tonnes, these days a big double-decker is only about 12.

The ideal size does not go down. You want the bus as large as practical in all cases. The difference in cost between a small and large bus is not significant (a little more full, and the bus costs 10x as much - but both of these are insignificant over the life of the bus), and the large bus can handle it if you decide to take everyone bowling after your kids birthday party or some such.

The dominant constraint is usually road capacity during peak hours. You need a certain number of large buses to handle that, and those buses are cheap to use outside peak hours.

Ideal bus size may go down if peak demand also decreases. For example, if working from home becomes more common. Smaller buses may also become more viable when direct bus lines are replaced with rail lines and feeder buses.

If you punt that means someone isn't picked when expected and so is late. Even spinning a new one up means they are late. Geometry works againts you