It might not make sense on the macro scale, but it certainly does on the micro scale. Even if you only use the extra size of the car a fraction of the time, the amount of money and time you would spend to rent a larger vehicle the times you actually need one would be much higher than the premium you're paying for having a vehicle with a capacity that well exceeds your average use case.

Also, one out six Americans are over 65 years of age, two out of five are obese, so the "extreme vast majority" being neither old nor disabled is really a question of definition.

Old people whose kids have left home should be a great demographic for smaller cars. There is a dead comment here that blames this on a vicious cycle of feeling unsafe in smaller vehicles next to giant pickup trucks. To me that's a big part of the reason.

So old people who’s kids have left never have kids come back to visit, or grandkids, or a 30+ lb dog?

As I said in another comment, if we had more easily available rental cars that would be more commonly used outside of a vacation context, we could buy cars that match the need we have 98% of the time, rather than optimize for the 2%.

I also think that this is another corrective action problem. The benefit to everyone from having most carsm in our cities be much smaller would be huge, but the inconvenience from having a smaller car is the thing an individual can control.

the real metric here is miles not ownership. I have a large truck because I need to haul building materials and a trailer. there is no way I am renting

I don't think anyone is saying that nobody at all should have a pickup truck. Of course some people need it to actually haul large equipment or materials. That seems to be <50% of those with those large vehicles though. At the same time there also seems to be a somewhat vocal group (not sure if large) that needs pickup trucks for work, but find that the design of current pickup trucks favors aesthetics over practical benefits with beds being too high to load comfortable, too much space being taken up by the cabin etc. This thread had some very interesting discussion of those issues: https://news.ycombinator.com/item?id=35645624

Again, there are legitimate use cases for a lot of this. Some people even need a semi ;)