That is possible to compress on a cartridge? And CD Project Red didn't seem to interested, if they've done it, they must've outsourced it to a porting team. Which one? Witcher 3 ported by Panic Button would be something.
Switch cartridges go up to 32GB with 64GB due to be made available, on PC where full textures are used and compression isn't utilized as much TW3 is 50GB compression isn't really the issue tbh.