> The value of scroll-bar-width is used only in the first frame.
 >
 > 1.) Create file /tmp/init.el:
 >
 > --8<---------------cut here---------------start------------->8---
 > (set-face-attribute 'default nil :height 150)
 > (setq default-frame-alist '((scroll-bar-width . 10)))
 > (make-frame-command)
 > (print default-frame-alist)
 > --8<---------------cut here---------------end--------------->8---
 >
 > 2.) Start emacs:
 >
 > emacs -Q -l /tmp/init.el
 >
 > 3.) Compare the widths of the scroll-bars.

Should be fixed with revision 116894 on the Emacs-24 release branch.

If you are on trunk you will either have to wait until the fix
propagates or try the attached diffs.

Thanks for the report, martin