I've recently been reading my way through the excellent Dresden Files recently, and noticed that every book was getting fatter and fatter. This seems to be quite a common trait among book series, particularly those of a fantastical nature. I certainly remember Harry Potter and The Dark Tower growing in size, at least for the first four or five books anyway.
So I did what any reasonable person would: Thought of 10 or so series of fantasy novels, counted up their pages (using a technique of counting known as "Consulting Wikipedia or Amazon") and generated a lovely bar chart. Here it is! Click it to make it bigger!
Erm, so, as you can see, there is a definite trend towards books getting fatter as the author grows more confident and decides to see what they can get away with. There are some complete misnomers such as The Vampire Chronicles by Anne Rice, but I think there's a general trend towards growth! I'm too lazy to do any further analysis, of course. Here is the spreadsheet if you're interested.
Oh, books are in publication order, and I've only looked at the first 10 novels of a series in order to keep the chart looking pretty.