Wow. That much just in Japan? Most games cannot manage this worldwide.
Some people seem to have expected more, but this is a great surge from the first game.

Wow, best selling Switch game day 1!!! almost 700k, and it didn't even get a full 7 days or include digital sales, 1 million in a few months seems likely.

I got this info from another site. Don't know if its correct, as in, if its from first 3 days or a full week.

Launch sales comparison:
Zelda BOTW: 187,000
1 2 Switch: 80,000
Mario Kart 8 Deluxe: 284,000
Arms: 100,000
Zelda BOTW + 1 2 Switch + Mario Kart 8 Deluxe + Arms: 651,000
Splatoon 2: 671,000

Spl2n is not "most games", it's the successor of a game that sold 1.5m in Japan and ~5m worldwide on the WiiU with it's tiny user base and that the fw sales of a 2nd entry are much higher than that of a 1st entry is very standard aswell,  so these fw number were in fact expected

but as the Switch will very likely blast past the WiiUs total install base with ease and as the game is still very fresh and more refined in the 2nd entry, Spl2n has a lot of potential for growth - 3m Japan/10m WW (2x Spl1) is a possibility

Why did people expect this to sell 800-900k first week? It doesn't happen with small install bases. This is incredible though for an install base of 1.2 million!! WOW

Splatoon 2 had insane sales, not a doubt about it, but Switch could have sold maybe 150k and in that case Splatoon 2 would probably have sold even more, but oh well, sooner or later the people that wanted a switch and couldn't get it will get it, so who cares.

Because of the number of COMG points pre orders. The question is how many bought splatoon 2 but still not have a switch?

Good first week sales and Switch hardware sales. Of course, the real test will come in the following months: Will the game have tentacles or drop off in sales after a few weeks? With the continued updates it should sell well over a long period of time.

On a more depressing note, it looks like Nintendo wasn't able to actually ramp up production yet. Instead they shipped 25k units instead of 40k for some weeks and now put them all out there at once lol.

If Splatoon is any indication, Splatoon 2 will have very strong legs, after launch Splatoon 1 was around hole year constantly in top 10 selling games in Japan, and we talking about about game for Wii U.

Sources saying that Nintendo shipped around 30k regular Switch units and around 70k Splatoon bundles, so if next week is again around 25-30k, that means they are still on around 30k weekly basis.

But COMG gives an indication only and for recently launched hardwares it doesn't correlate with usual numbers (even different games and platforms skew from the "norm"). These numbers for Splatoon are actually incredible