Nintendo Switch cartridges can come in varying sizes to accommodate the game size. However, it seems WB Games has reportedly gone with the cheaper smaller cartridges to save on costs. Thus, the user will have to download the rest of the game, which is 13GB.
Nintendo could have a difficult decision to make here. Do you potentially scare away publishers by forcing them to use larger and more expensive cartridges, or do you put it on the customer to pick up a SD Card and potentially damage your systems reputation?
As for how you prevent that from happening, Nintendo could enforce a maximum download size for the first week of launch. I could see this becoming ugly fast.