I'm bringing this thread back to life because I seem to have a similar issue and I'm not entirely positive of how to correct it. In my library I have all my book with a cover image that matches size 1264x1680:
However, upon sending the book to the device (a Kindle Oasis) the cover art size is reduced to 376x500:
I'm not sure what's going on here or what I'm missing. I ran the Welcome Wizard and have selected the Kindle Oasis. Any thoughts on what's happening? Appreciate the help!!