As you can see, the most popular teams from large states, like the Spurs and Lakers, cover a lot of ground. Surprisingly, though, it might actually be fandoms of the Bulls, Raptors, Nuggets and Timberwolves that are most far-reaching (although the latter three obviously don’t register near the top when it comes to total number of fans in general).
Unfortunately for teams like the Nets, Clippers, Kings and Magic, they play in the same state as a much more popular team (the Warriors do too, but they seem to still have a decent stretch up Northern California to avoid the wrath of Lakers fans), so they get only a small little blip on the map just to show they exist.
It’s a pretty interesting graphic that seems to be mostly accurate when you really break it down. However, there is obviously a ton of unclaimed land in the States which tends to skew to either the generically popular teams like the Knicks, Lakers and Bulls or recently successful franchises like the Warriors and Heat. Still, while those teams all certainly have their fans in the unclaimed territories, Reddit user BCP27 knows who really reigns supreme, and he helpfully fixed the graphic to make it as accurate as possible.
In addition to his work here, jfreedo also recently put a similar graphic on Reddit showing the fan bases of every NFL team as well, which you can see here.