This could scarcely be less scientific. Many of these teams have moved, cities have changed size through the years, the huge-market but second-tier Clippers and Nets skew things, and I shoehorned Toronto into U.S. rankings in non-sensical fashion (note: both Dallas and Toronto are listed as fifth-biggest). Not to mention, other people have done this analysis better elsewhere.
Turn a blind eye to all that and you'll see that through the entire history of the NBA the ten most successful teams come from, on average, smaller markets (average rank: 21.4) than the ten worst teams (average rank: 20.1).
Even if the analysis is very rough, it's almost undeniable that the Lakers and Celtics are powerhouses not merely because of market heft -- plenty of teams in huge markets have struggled to compete.
What's the point of this? I have heard from several fans who despair that their team has no chance because the NBA makes it impossible for small markets. The people in San Antonio, Phoenix, Salt Lake City, Portland, Oklahoma City, Milwaukee and Orlando argue otherwise.
You might say those teams are outliers who found themselves with players like Tim Duncan, Dwight Howard, Kareem Abdul-Jabbar, Karl Malone, John Stockton, Kevin Durant, Bill Walton, Steve Nash and the like.
And I'd say that's a heck of a lot of outliers.
The bottom line is that if you can get your hands on special players, you have a chance, no matter where you are.