"A microSD card will be needed for certain Nintendo Switch games that contain an especially large amount of content and require additional storage for players to enjoy the full experience.
If you purchase a physical version of a game that requires an additional microSD memory card, you will be able to play a portion of the game right out of the box (for example, specific levels or modes). To enjoy the full game, downloading additional data is required. Depending on the storage requirements for each game, it may be necessary to purchase a microSD card to expand storage space. When purchasing a digital version of the game, it may also be necessary to purchase a microSD card depending on the games storage requirements and the storage available on the consumers Nintendo Switch console."
http://m.ign.com/articles/2017/09/01/some-nintendo-switch-games-will-require-memory-cards
Edit: looks like on this specific case, the game is not bigger than 32 GB. They just decided to use a lower capacity cartridge to save costs.