Square-Enix did a lot of work to squeeze Dragon Quest XI S onto the Switch. They managed to take 30 GB PS4 file size and work that down to 13.5 GB on Switch. How in the world did Square-Enix pull that off? The company shared a tiny bit of insight on the topic in a Famitsu feature.
Square-Enix ended up reducing the file size of all assets tied to the game. Obviously there's a number of different steps that had to be taken with these elements, as some things are harder to shrink down than others. Still, the reduction in file size required for each aspect of the game let them shave things down all the way to 13.5 GB on Switch. This also had the added bonus of making the Switch version load faster!