This is only for one university (Uni of Minnesota), but interesting none the less. I would be interested to see if this holds up across universities in the US and even in other parts of the world. In this, history grads produce the longest dissertations (keep in mind that dissertations are front page only, and tend to have specific page width, etc). The point is always to make your argument, not to write as many pages as possible.
Here is a link to all the number crunching fun: