You're the first person to say Amazon's update files don't work.
Note that they are incremental updates (Amazon first started shipping full firmware images ~5.3.x IIRC) so it is natural for them to be quite small.
Have you tried them? (Or did you assume such small files won't work and just give up?)
Perhaps the download was corrupted or blocked. If you open the file in a text editor, does it perhaps look like HTML?
You can use KindleTool to check the validity of a firmware update:
https://www.mobileread.com/forums/sho...d.php?t=187880