While the size reduction is impressive, "highly compressed" does not come without compromises.
NKIT is a specialized utility designed to shrink Nintendo discs to their absolute minimum size for archival purposes. An NKIT version of Mario Kart Wii is incredibly small, but you often need to convert it back to standard ISO or WBFS to play it smoothly on certain emulators. How to Play Mario Kart Wii Highly Compressed Mario Kart Wii Highly Compressed
If you are running a library of hundreds of retro games on an SD card or a small SSD, every gigabyte counts. While the size reduction is impressive, "highly compressed"