On default it is controlled by the body rule: body, body.mceforcecolors. You can override it by using id #tinymce or class mceContentBody on the a body tag instead. It will cause issues with the full editor as well, though.
That fixed it. I was looking for the template that controlled this, and overlooked it due to frustration, editor_contents.css.
Thank you Magnus, much appreciated. Now I think I have XF converted over for Part 1 of responsive design, being the entire font setup is now controlled via ems. No more pxs or pts used. All fluid text for maximum effect across all devices.
That editor was driving me nuts, as I had to reset XF's default body font from 13px to 100%, then reset it based on browser 16px into ems. So now regardless which device it hits, it will be standardised based on a completely fluid font set.