TL;DR
Untouched nand will compress. Written To nand will not
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In my experience the first clean nand backup I did compressed down to 1.5gb
That was on a a virtually clean nand. No games installed to nand and maybe one or two game updates and a few screen shots.
My most recent back up did not compress down much at all.
This is because I had utilized most of the nand previously for games and updates etc.
Even though i had deleted said games and showed I had most of the nand free it still did not compress down.
The reason for this is because my first backup had 'virgin' sections that essentialy had never been written to so could be compressed down to next to nothing.
Fast forward to my recent backup although I have gigs of free space Its no longer 'virgin' space and has physical data on it that is no longer easy to compress.
One way to make back up compress down to a low level is if you remove all game data etc and a dev makes an app to scrub unused space to a format that will compress down.
Something along the lines of wiiscrubber back in the day. As Wii iso where full DVD size bit probably only utilized 40% or less of the disc (game dependant). Even when zipped it would hardly compress.
Run the iso through wiiscrubber all unused space was converted to data that was easy to compress. Zip game down to maybe 700mb unzip to full size burn and the Wii did not care.
Something similar to rom trimming however rom trimming removes the data instead of changing it.
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