One thing I neglected to mention when moaning about book length is font size. Generally speaking old books seem to contain smaller typeface which obviously helps contribute to their shorter page count. Although there are exceptions to this. A year or two back I read a David Baldacci novel which was over 600 pages long and had a typeface so small you'd have trouble reading it with an electron microscope. I had a headache by the time I finished reading that bastard.
Another important factor when it comes to long books is writing style. Some long books seem shorter as they have a breezier style. I recently read John Connolly's The Unquiet and found it a joy to read, much easier to digest than his previous novel in the Charlie Parker series. Although admittedly I wasn't in the best of moods when I read The Black Angel which might go some way as to explaining why I didn't enjoy it as much. But anyway Connolly claims that although a lot of people found TU the quicker read it is actually the same length as TBA. (My copy of TU seems to be shorter but that may be due to the difference in page size between paperbacks and hardbacks.) Connolly says that he took a different approach to writing the two novels as TBA was an epic novel full of metaphysical conceits and a sprawling plot that spanned the decades plus two continents whereas TU was a much more intimate affair. Therefore TBA just feels longer.
Coincidentally I recently read an old interview with Lee Child and he said that for his first novel The Killing Floor he deliberately avoided using long sentences as he didn't want to do anything to alienate his readers. He was trying to sell the novel before his redundancy money ran out and he didn't want to do anything to jeopardise his chances. Once he sold the first novel his style seems slightly less clipped but based on the novels I've read he's still making sure that you don't need a degree in English Literature to understand his prose.
And legend has it that James Ellroy created his staccato pseudo-beatnik style after the manuscript for one of his novels was too long and his agent advised him to cut out all the unnecessary words. This allowed him to cram tons of plot into a single book.
Of course if had been a Fanatasy novel his agent would have told him not to change a single word. They'd just cut the book into three sections and sell it as a trilogy.