Change the length of scrollbar arrow buttons
 

Change the length of scrollbar arrow buttons:

To go with changing the thickness of the scrollbars, you may wish to change the length of the scrollbar arrow buttons...
For the horizontal (bottom) scrollbar arrows:
HKLM\System\GWE\cyVScr = 9 (DWORD decimal)
For the vertical (right) scrollbar:
HKLM\System\GWE\cxHScr = 9 (DWORD decimal)
The default length is 13. You can set the value to pretty much anything you like