You might be also interested in the talk that is going on this thread - Extra compress kra files for archival - #38 by brunoais which asks for further compression and also has some nice tricks to archive the kra files.
Sidenote: Regarding PSD format when I used to use photoshop and me and my colleague wanted to save space when sending files etc we saved the psd files hiding all the layers. We used to put everything in a toplevel group and hide it and then save it. This would of course remove preview which was not available anyway at that time. But it saved considerable size. Then we moved on to tiff with lzma compression and it too when saved way less when we hid all the layers. You can see that krz does something similar by removing the preview 