Also the margin is irrelevant for a vehicle's danger to pedestrians or its consumption, only its mass and velocity (because the energy of a moving object is proportional to the mass and to the square of the velocity), which is why even a bicycle can be deadly to a pedestrian if going at a high enough speed.

Let me put it this way: there's a reason why cyclists and pedestrians can safely coexist on multi-use trails without so much as even lanes or stop signs at trail intersections, but adding even little kei cars to that mix would be a ridiculous disaster.

If being run over by a kei car is already enough to kill you anyway, then it's not as if being run over by a big truck instead can somehow kill you "more." And sure, there's some small difference in probability, but in terms of danger to pedestrians, a kei car is way, way, way closer to a big truck than it is to a bicycle.

Also, again, every car, regardless of size, takes up a whole parking space (even kei cars in places outside Japan that don't design special small parking spaces for them). Every car, regardless of size, takes up a whole travel lane width. Every car, regardless of size, takes up the essentially the same length of road while in motion (because spacing between cars is dominated by minimum safe following distance, and the length of the car itself is negligible in comparison). Every trip taken by car, regardless of car size, is one not taken by cycling or transit. Every person who owns a car, regardless of car size, needs somewhere to store it and is therefore contributing to demand for parking spaces/loss of walkability.

The negative effects of cars on walkability and urban design are absolutely dependent on their number, not their size.

I can get it if your detesting of cars is an absolute thing with no specific reason, but I suspect that for most of us our detesting of cars is anchored on various very concrete reasons, and personally danger to pedestrians and other road users such as cyclists and polution are two of the biggest ones for me, in which case it makes sense to detest even more a trend in car use that makes them more dangerous and more poluting

Okay, but you need to understand that both of those problems are also absolutely dominated by the sheer number of cars, not the size of them!

Going from a 55mph road with no shoulder with big trucks zooming down it to a 55mph road with no shoulder with kei cars zooming down it would hardly be any safer for cyclists at all. In contrast, adding separated bike infrastructure to such a road would make it way safer for cyclists, even if the cars using it continued to be big trucks.

Going from a big car to a small car might be up to about 50% more efficient (and that's making a very generous assumption) -- and that margin applies equally to going from a big gasoline car to a small gasoline car, or going from a big electric car to a small electric car. In contrast, going from an electric car to an e-bike is 20 to 35 times more efficient. Even the smallest cars still weigh thousands of pounds; there's a limit to how efficient they can get. The big wins in reducing pollution are achieved by getting people out of cars entirely, not by getting them into smaller ones.

Attention/political capital is a limited resource. Spending any of it on reducing the size of cars instead of their number is borderline misguided and unhelpful, in every case.