The Japanese-made trucks have gained a cultlike following, attracting small-business owners seeking affordable alternatives to traditional pickup trucks.
Been to Japan lately and can share some photos. There are even Kei Fire Trucks, for the many small roads with wooden houses and shrines etc.
And then there are hundreds of different kei truck and van types for all purposes, even concrete mixers.
Also, private houses in cities are often small and space-saving and so are the cars. A sensible use of public space – and cars only park on private property or rented parking spaces.
Neither the fire truck nor the concrete truck are Kei class vehicles.
They are small diesel trucks, yes, but Kei literally means 'light' and have strict weight limits on both the weight of the vehicle and how much load they can carry.
Well, those are also not vehicles that the average citizen buys. They're specialized for their purpose, the fire truck needs to transport a decent amount of water and 4-5 people, and concrete is heavy stuff. But in a certain way they follow the same design philosophy.
Yes in some neighborhoods and villages the roads are so narrow that they can practically only be driven on by Kei cars.
Japan as an Island has limited space available for natural reasons, plus large parts of the country are mountain area. So the old cities have been built in plains and reached high density. Building is strictly regulated.
And that has also grown into the culture. The Japanese sense for efficiency is legendary and so you simply don't waste space. And in general, you don't show off with oversized cars. Understatement is part of the general habitus. Shintoism and Buddhism have deep roots and that certainly plays a role too.