Why is this trending? That's old news, and unlikely to be valid. The original source is a minor blog-like website that ranks #5,357 on Alexa in USA alone. This information has not been verified since, and while 16GB certainly sounds reasonable, it is far more likely that Game Cards will come in various sizes. There is no point having only 16GB cards as many games will not need as much. Likewise, it should be absolutely expected that certain games will come in larger 32GB sizes. It is very doubtful that we'll see 64GB Game Cards however -- the costs wouldn't justify it and it would hardly be useful for a system like Switch.
Let us not forget that the vast majority of games on PS4 and XB1 have 'padded' data, duplicated data to ease up on loading times.
Given Nintendo's history, in fact given Nintendo's entire history of using cartridges, they will employ variable game card sizes depending on the requirements of the game developer. Smaller size cards will be cheaper, while the largest cards will be more expensive for the producer/developer.
Game Boy: 56kb - 2MB
Game Boy Colour: 256kb - 16MB
Game Boy Advance: 4MB - 32MB
DS: 8MB - 512MB
3DS: 128MB - 8GB
3DS launched with '2GB standard size', so for Switch I'd expect game sizes to range from 2GB to 32GB.
