Comment 2 for bug 2004522

No, the editor never saves a file unless you ask it to. What does happen by default when you run the editor is metadata and cover is updated in the file, to match what is in your calibre library. which will be why the file size increased. You can disable this behavior in the editor preferences under calibre integration. As for why send to kindle is failing with it, only amazon can tell you. From my experience I can tell you the amazon's epub intake is extremely buggy and fails on all sorts of valid files. It's even in the calibre FAQ: https://manual.calibre-ebook.com/faq.html#amazon-is-stopping-email-delivery-of-mobi-files