This is the 2012 revenue generated by each team (LINK HERE):
http://static4.businessinsider.com/image/5154a429eab8ea8670000014-608-457/untitled-2-200.jpg
The Sox, at $336mm, are a top dog in terms of generating revenue, and that's with a park that fills ~10k less than most other ballparks.
There's just a ton more interest in Boston than other teams. As a700 pointed out, revenues coincide with the term "big market". This is a perfect example of that.
Without looking at the graph, I would think the Sox, Yanks, Phils, and Cubs have the biggest markets, and that's confirmed by the revenues generated.