Change metapage command to switch text size only locally

Hi, I just discovered that the explicit change of the font size in the \metapage commands affects the remaining text.

I use \includes for the different parts of the frontmatter (and chapters etc.) and improve single chapters by means of \includeonly for faster rendering. Then I discovered that a chapter was warning-free if I only compiled the chapter alone (or the whole thesis without the meta page), but 4 underfull vboxes were reported if I also include the metapage.

Unfortunately, I have no in-depth understanding of what's going on, but could avoid this problem with the change of this merge request.