Nintendo has the best texture compression wizardry in the biz. This game is smaller than MK8DX. Only 5.7GB. The save file alone from NBA 2K18 is 5GB. Minecraft's save file is 2GB. Part of what makes SMO so small is that its only running up to 900p and reuses assets and textures throughout the game. Does not have prerendered cut scenes because it uses in-game assets to generate its cut scenes and it does not require high quality voice acting files or super high quality textures as well.
I'm downloading this game straight to the Switch 32GB since that's how you get the fastest load times for games, 2nd is MicroSD card and 3rd is cartridge.