I've been having some font size weirdness with the inserted metadata page, where sometimes it comes out with a huge font. I'm really not sure what causes it, but I think it might be something to do with the "Source base font size is" line.
I've changed the book jacket stylesheet to remove tags and ratings, but I doubt that's related, as most of the converted books are ok. I'd thought it was happening more often with my epubs from Kobo, but it doesn't happen there all the time. I've definitely noticed the problem more with books from Samhain Publishing, and the attached image with the really big font is one of those. I've had it happen with a few of my amazon azw3 books as well. I also wondered if it was connected to using that plugin we don't really talk about, but I don't think it is, as I've had it happen with books that were completely drm-free to start with. I've also run lots of books through the plugin and had the metadata page come out fine. The attached Death by a Honeybee book jacket is one, and I attached it as an example of how it's supposed to look.
I was trying to find a public domain book that had the problem so I could attach it, but no luck so far. I did however find some that are free on kobo, so I hope it's ok to link to those.
This one gives a book jacket with big font, and
this one gives one with gigantic font. I've also attached the conversion logs for these.
I hope I've provided enough info to help fix this problem, as the metadata page is one of my favorite features (which is really saying something, given how useful Calibre is). Thanks so much!