MerricB said:
Newer authors tend to use more pages and tell bigger stories.
I don't entirely disagree with this, but wanted to point out that there are plenty of big, long stories in earlier years: Peake's Gormenghast books require serious commitment from any writer, and Charles Dickens (in which one can see many modern fantasy tropes prefigured) certainly wrote effective doorstops (in serial format, no less).
But 50, 100 years ago, I suspect the primary market for fiction was magazines. And so stories tended to be shorter, more self-contained. Nowadays the publishing business has its manufacturing and distribution processes well-tuned enough that churning out thousands of mammoth tomes and getting them to bookstore shelves and readers' hands is more feasible than it once was.
So part of the trend has little to do with fantasy in and of itself. And certainly other genres produce extended series -- go review the mystery section of your favourite bookstore and see how many books are subtitled "A Mr. Hoobajoob Mystery" or "Fifth in the Small-Time Murders series".
But of course it is fantasy that is primarily concerned with world-building. In writing a fantasy novel, a writer typically creates an entire world, with history and what not. When telling their tale, they have to also explain the context in which it occurs, which something the writer of detective novels doesn't have to worry about so much. When John Le Carre's character goes to France, John can write, "He went to France," and his readers have all sorts of built-in context for that statement.
But when a fantasy writer writes, "He went to Yamamunga," he needs to provide for his readers all the context that Le Carre gets for free.
And of course part of the motivation driving a fantasy writer is the chance to show off his world, and therefore a story that explores every nook and cranny and brings to light every clever detail is going to naturally appeal to him.
So I don't think it should be surprising that fantasy writers tend towards large tomes.
That said, it's instructive to review the old magazine/serial writers, because they tend be very good at providing that context very efficiently. You don't need to read much Leiber to know what Lankhmar is like, nor much Howard to get a very clear impression of Hyboria.
Or go back to Burroughs and watch how he evokes Barsoom. Mm, tasty...