A listing on Nintendo’s Japanese store shows that just 5.7GB will be required for the digital version.

Other Nintendo-published games are similarly small. ARMS for Switch is just 2.2GB, Splatoon 2 for Switch is 3.1GB, and Super Mario 3D World for Wii U is about 1.7GB. 

Source 1 and Source 2

That's fine.

Nice compression skills!! I wish other developers would follow suit.

Absolute witchcraft.

Same, I can't believe that it's normal now for games to be 50 GB and above...

I'm pretty sure the problem lies deeper in design, and not just in 'compression skills'. To be honest, in most cases, game size really isn't much of a problem, although it can often be an annoyance.

Quality > Quantity

Pff, Quantum Break takes up 123GB on my hard drive!

That wizardry

More western developers are used to PC development with hard drives and really don't take too much time to compress instead go for the best they can do in terms of visuals and etc regardless of platform. Nintendo and a few other (not all) Japanese developers usually build for their particular platform and are aware of limitations such as storage from the jump.

Going forward, especially if the Switch is a massive success, more developers may try to trim the fat but I am sure more then a few will still just do whatever and expect the consumer to meet the requirements of their game.