The "improvement" is that in Windows 11, you don't have to put the mouse pointer on a scroll bar in order to scroll an app. Just rest the mouse pointer anywhere on the window, and scroll the scroll wheel.
This is particularly handy when you are working with two (or more) open windows. Just move the mouse pointer onto another window and scroll that window with the scroll wheel. No need to hunt for a scroll bar.
I have to object to this reasoning. They could have done that without making the scroll bars so small.
And the scroll wheel is only useful if there is a positively *tiny* amount of scrolling to be done.
If you e.g. open a large log file in a text editor, you will soon see that using the vertical scoll bar is both much faster and less tiring.