OOjs UI uses font-size of 12.8px (0.8×16px) for windows and toolbars, but Vector skin uses 14px (0.875×16px) for body content, creating inconsistency between widgets in windows/toolbars and "free-standing" ones.
This has been recognized in VisualEditor, and corrected there (modules/ve-mw/init/styles/ve.init.mw.ViewPageTarget-vector.css), but not in the skin or OOUI proper. Incidentally, this is not a problem in MonoBook, which uses font-size of 12.7px (1.27×10px); however, its bizarre way of setting that size necessitates a different set of tweaks.
We should remove the `font-size: 0.8em` rules sprinkled throughout OOUI and instead set the right font-size for windows in each skin's CSS (unless the skin already sets a sensible font-size on `body`, which they all should but seemingly none does). This will be a delicate endeavor that must be synchronized, but it will also remove some technical debt that keeps breaking things (right now VE inspectors in MonoBook are being displayed with incorrect, massive font size, for example).