How large does a business need to be on a global scale before we can smack down bad actors for abusing it? If you are a street corner business and your competitors down the street sell things at a loss just until they can put you out of business, should that be allowed because you were only a local business and didn't have millions of customers?
It's a factor in whether "219.43 million people" is a lot of people. If that's 219 million people in the US (i.e. well over half the population), that's obviously massively more significant than 219 million people worldwide.
But for the question of anti-trust action it doesn't matter whether it's "a lot" of people, it matters whether it's an insignificant number of people - if there were 10 smartwatch users in the US, the argument "who cares about smartwatch users" could be valid, but it makes no difference whether there's 219 million people in the US or 219 thousand people in US, since even 219 thousand users is definitely much, much more than sufficient to justify intervention.
I don't really know about sourcing market data, but this[0] page cites Deloitte and Pew:
>The global smartwatch adoption rate has reached an impressive 21.7% of the adult population
...
>The adoption rate of smartwatches is expected to continue growing, with industry projections suggesting that it will surpass 25% of the adult population shortly.
I don't believe a fifth or a quarter of the adult population could rationally be called 'niche'.
