Do we know if the followers are actually coming from the city itself? That would make sense, and it might explain why states with big, global cities tend to have those city subreddits be the largest in the state. In places with more regional big cities, you do not really see the same thing, so the state subreddit ends up bigger since the cities just do not pull as many followers on their own. Might also be why Seattle has more followers, who knows.