The Wii was a monster of a machine sale wise. The console sold by far the most out of its competitors of the generation (the PlayStation 3 and Xbox 360), and that was largely due to how the system caught on with a more casual audience. In the end, it's estimated the console sold north of 100 million consoles.
Well, according to one analyst, the Nintendo Switch is doing even better. NPD Group's Mat Piscatella said that comparing sales of the two consoles in their first 10 month period, the newer console is winning. Posting on post-NeoGAF forum Resetera, he said:
Switch is on pace to exceed Wii by 20%+ over the first 10 months of sales.
The thing is though, the Switch, from a demographic and usage basis, is nothing like the Wii. It's much more a core gamer console than anything Nintendo has done since maybe NGC?
And because it's a hybrid, multi console ownership per household could be a thing, making upside potential bigger than the Wii.
If the Switch really does outsell the Wii it would be quite an achievement. Don't pop out the champagne yet though. The Wii had a very long tail and really exploded in popularity in its second year. It remains to be seen if the Switch will continue to find new audiences in the same way; I wouldn't count that out as a possibility though.