I think this is where we get to eventually when there are no more human drivers and so the roads become extremely safe. Until then, you need a certain amount o

I think this is where we get to eventually when there are no more human drivers and so the roads become extremely safe. Until then, you need a certain amount of mass to protect you from idiots, no matter how safe the computer is driving. Given that mass, adding three extra seats is little additional cost, and gives you additional flexibility in dispatch (reducing wait times). But to your point, they will probably have a "large" size pretty soon, a la Uber XL (although it will only come after the first no-steering-wheel vehicle is widely deployed).