The scroller for charts is special - we coded it specifically for charts, it has functionality that is specific for charts (like being able to drag the edges) etc.
The other scrollbars are generic OS scrollbars. Their widths are set by the OS, globally, for all programs.
Here is how to change those for win10: http://winaero.com/blog/how-to-change-the-size-of-scrollbars-in-windows-10/
For Win7, go to you Control Panel/Personalization/Window Color click "Advanced Appearance Settings", set Item drop box to 'Scrollbars" , set Size spin control.