> Why does this API keep increasing the font size? With a selection, try
> the test command.
>
> Because the factor of 10 is wrong, AFAIU.
>
Factor of 1 decreases the font size.

> Alternatively, what's the best way to query the font-size at point and
> re-apply using face property?
>
> I don't understand: if you don't want to change the height, then why
> do you have :height in the face spec at all?
>
> In any case, :height could be a relative value of 1.0.
>
 I'm trying to increase/decrease the font size in the selected region. So I
need a base value to work with.