Ok, but how does that impact the characters sizes being different?
> I came across something that feels a bit odd in tab-line, maybe you
> can explain it.
>
> I've been playing around with tab-line-format and noticed that what
> gets displayed in the tab-line actually differs based on whether you
> are using the `tab-line' face or not.
>
> See the attached picture. Both Emacs were launched with -Q. Although
> the face definition is the same in both situations the result is
> different. Specially the rendered size of the characters.
This is how mode-line/header-line/tab-line work - they always use
hard-coded faces with names 'mode-line', 'header-line', 'tab-line',
so you can override face definitions only by customizing these faces.