[NiLuJe]

@purplepancake: If you end up with smaller covers and a PDOC banner, it means that your files don't have an ASIN set (exth 113) in the metadata, which, if you're using Calibre to build your mobi/azw3, means they were built *WITH* the 'Enable sharing' setting checked. (If those files were built by KindleGen, on the other hand, then it's perfectly normal. Convert via Calibre, of tweak the metadata manually to add an ASIN [I think there's a plugin for that, used by some Fire users [It's a feature of the Quality Check plugin, I think?]]).

Check that your files were built properly (*without* 'Enable sharing') (you can even go so far as to check the metadata of the files on your device with MobiUnpack if you're still not convinced), and that you're using Calibre's "Send To Device" over USB in a recent Calibre version.

(Don't forget the delete/unplug thing if you're doing this on the same books over and over, the Kindle gets confused (and breaks the thumbnails, basically) when you switch files without letting it know they were gone first).