I was having a conversation recently in which I tried to defend "old" novels--the conversation was fairly brief, but still it put me in mind of one trait that makes older books more difficult for the modern reader. I'm not talking about vocabulary, sentence structure, or the overall long length of Victorian novels. I'm thinking of chapter length.
After breezing through a small handful of modern novels this summer, I have just started rereading Charlotte Bronte's The Professor (I plan to reread Jane Eyre and Villette next, too: I'm preparing to do my honors thesis on these three novels this coming year). Most of the chapters I have read so far haven't been too long, but it made me remember that it isn't always so. If you pick up a Victorian novel, you can easily have chapters of thirty dense pages (as in, pages with much smaller print than in How It Ends). It's difficult to commit to a long portion of reading in one sitting, and also is it difficult to stop right in the middle of a chapter. (Or try reading Moll Flanders, which doesn't even have chapters, but is instead just one long narrative.)
True, some "older" books may have breaks in chapters, even if the chapters themselves are long. If you count The Lord of the Rings as old, there are certainly breaks (the extra space in between paragraphs that separates scenes), though there are also some very long chapters (hello, "The Council of Elrond"). Breaks like this make it a bit easier: they provide at least some sort of potential stopping point.
All of this is contrary to most of the books published today, where a chapter may be ten or twenty small pages, something you can read in half hour. It's easier to say, let me read just one more chapter, when you know that chapter won't take you an hour or two to read.
The "problem," if you want to call it that, may not be in the thickness of certain books. It's in the thickness of individual readings: it's often easier to have a long-term attention span (taking a month or two or three to read a book) than a short-term one (taking three hours to read one chapter).