The plot below shows the attendance rate. This is basically (Attendance of Game) / (Max Attendance of season). The closer to 1, means the team is closer to capicity (for that season of course). As one can see, the Yankees and Red Sox's sell out most of their games, and they do this regardless of their winning percentage. That is because (or so i think) since the beginning of time these teams have been winning and have a very large base of dedicated fans, but i wont go into that. The more interesting part to look at are the Nats and Rays, both small market teams (second graph shows just those two). These two teams are much more volatile compared to the heritage that is the Yankees and Red Soxs.
What is even more interesting is their winning percentages. Notice how they both have two extremes. The Rays, who qualified for the playoffs by winning (debatable the hardest) their division. The Nationals on the other hand, got more fans to come see How Do You Know at park than the Nationals.