They share the style setting because the toolbar icons and tool window tab icons share design elements. It would just create an inconsistent mess to let them have different styles.
Small for toolbar icons is 16x16 (as it was in earlier releases).
Small for tool window tab icons is 20x20, and extra-small is 16x16. We could have called 16x16 "Small", but then I'd have to use a contrived term like "Medium-small" for 20x20.
I had to create the artwork for the 20x20 size manually, which was OK for the tab icons, where having that size was more valuable, and there were only 47 to draw. Not for the 359 icons (times 3 styles) used in toolbars. Also, the 20x20 size is not as valuable for toolbars as it is for tabs.