Why are some books so long?

Is it me, or has the average length of a novel gone up? I don't know if I'm reading different books than I used to, but it seems like most books I read nowadays are over 400 pages, and it's not rare to pick up a book that's over 500 pages long.

The length of a story in itself isn't that important to me. Some of my favorite books are over 400 pages. For example, my edition of Tolkien's The Fellowship of the Ring clocks in at over 500 pages, and it's one of my all-time favorites. However, what b...