Changes like these usually lead to the question where the current value comes from and why the new value is the right thing to do now.
I had a look at Windows 95 (thanks to pcjs.org), and it seems it has an 8 pixel minimum slider size. This is still the case on unthemed Windows XP ("windows classic"). The standard Windows XP theme makes it slightly larger, but not by much. Windows 11 on the other hand has a noticeably larger minimum slider size. All those were tested in plain notepad.exe.
My 2c is that the larger slider is a usability improvement on modern screens. It is easier to grab with the mouse. The smaller slider probably made it easier to place the scroll position accurately on low resolutions like 640x480 or 800x600, which were common during Win9x times.