The font and fontsize used for texhtml, as defined in MediaWiki:Common.css, was determined by comparing common default fonts found on Windows, OS X and Linux and is scaled to 118% to match their x-height. However, not everyone uses the default fonts. If you find that the rendered math is not of the same size as the surrounding text, you can adjust this in your personal CSS. For instance, the DejaVu Sans and DejaVu Serif fonts do not need scaling, in which case span.texhtml { font-size: 100%; } will restore proper display.

The texhtml classname is a remnant from the TeX renderer, which had a user preference to render TeX-written formula in "HTML when simple". That option has been removed some time ago, but the classname continues to be used for formula written in HTML.