I'm still mind blown how they managed to fit all of Breath of the Wild on that dinky thing even on the Wii U.
I'm hoping Metroid Prime 4 isn't compromised by this space.
Many Nintendo games are small because of the way they design their games (though Breath of the Wild is 13 GB before DLC/patches, which is larger than a lot of their other first party games).
Breath of the Wild (and many other Nintendo games) have very little voice acting compared to similar style open world games. In BotW mainly just the cutscenes are voiced, not the random dialog from NPCs. The lack of recorded dialog (which often comes in multiple languages) save a lot of space.
In general with many Nintendo games the cartoon art style they choose to use allows them to use fairly simple textures in some places (sometimes only a couple of colors with simple patterns).This can save space by not requiring a more complex texture with a lot more colors. They also can probably get away with lower resolution textures in a lot of cases as well.