Sounds great in theory but in reality, this is horrible advice taught by petty online marketers.... I have never heard a billionaire or a super millionaire say find the smallest possible market. They always say to do the opposite.
My experience is in the Biggest markets, the markets that many are afraid to enter. Markets with a few million searches per month and that's just the online reach.
I have been full time without a job for almost 7 years now, I have tested small tiny niches and they SUCK big time. There is never any decent money in these little dinky niches and the income never lasts.
Tiny little markets are easily saturated, 10 people enter your niche and your out of business. Plus you have to rely solely on SEO traffic and one dumb little long tail keyword that delivers 20 visitors a day haha, this is laughable.
There's a reason nobodies in that market and it's hardly because it's untapped. Untapped is just a fancy word people use to get people to buy stuff.
Your much better off to be a small fish in a massive pond, because even a small fraction of that income will be more than if you dominated some tiny niche (which if you did, I can guarantee you it will only be temporary)..
There is a publication I advertise in that has a circulation of over 750,000 people a month.
That's just one publication, there are literally hundreds more I can advertise in with numbers much larger than that.
I dare you to enter a big market, play with the big dogs, you'll not only have to get seriously better at marketing which will give you an edge over the wannabes and nobodies, you'll also have a bigger pool of customers to cast your nets into