Why is the scroll bar in Windows 11 so narrow? In what world is this an improvement?

As I said, I think this problem appears specifically on my system because of the 4K displays. The items in settings > system > display do give you control over the size of text and icons, etc, but I don't have any problem with these - they are set at 200% and are fine that way. The settings do not give you direct control over the width of scroll bars. It is possible to specify this in the registry, but not all programs seem to use this registry setting. Changing it affects some programs, but, as it turns out, it doesn't affect the programs that are the most problematic.

I like the new scrollbar size, I think the size fits better to the size of the other elements on screen.
If you have problems with the size of elements including the scrollbar you can also use the scaling feature (in settings -> system -> display) to make everything bigger.