It seems like Warner Bros is following Bethesda’s way of adding additional download files on their Nintendo Switch port. Base File size of Mortal Kombat 11 is 6.54GB on Nintendo Switch, but additional day one patch update is of 15.9 GB.
huh, base game is only 6.537GB, update is the bulk of it
gotta love @WBGames fuckery
— Nikki™ 🌹 (@NWPlayer123) April 9, 2019
This is likely that Warner Bros wants to use the 8GB game card rather than 16GB or 32GB because of the former one being cheaper. So, they convert the necessary files of 15.9 GB in day one patch.
It is a shame but it seems to be a common practice with Bethesda making users download all of Wolfenstein Youngblood in the game card. Even Capacom made players download whole game in the Mega Man X Legacy Collection!