Followers of SF and fantasy will have noticed this — most novels/stories are big fat tomes or are split across trilogies (I am currently reading The Tawny Man trilogy by Robin Hobb after having read The Farseer Trilogy and The Liveship Traders trilogy — and yes, the three trilogies are related) and quartets and what not. It is almost as if a SF & F writer can't tell a tale in a slim volume. An interesting explanation for the length of a typical SF & F novel is posted by science fiction writer Charles Ross:
Until the early 1990s, mass market SF/F paperbacks in the US were primarily sold via grocery store racks, supplied by local distributors (400+ of them). The standard wire rack held books face-out, either against a wall or on a rotating stand. And that's where the short form factor novel became established. Thinner books meant you could shove more of them into a rack that was, say, three inches deep. Go over half an inch thick, and you could no longer fit six paperbacks in a 3" rack. And there was only so much rack space to go around.During the inflationary 1970s and early 1980s, prices of just about everything soared. The publishers needed to increase their cover prices to compensate. But the grocery wholesalers who sold the books insisted "the product's gotta weigh more if you want to charge more". They weren't in the book business, after all, so just as buffalo tomatoes got bigger, so did paperbacks. (Even though this meant there was less room to go round in the wire racks.) You can only get so much milage by using thicker paper and a bigger typeface; so they began looking for longer novels.In the 1960s, an SF novel was 60-80,000 words, with 80K being considered overblown and long. By 1990 they'd grown to 90-100,000 words. Luckily the word processing revolution came along in the 1990s, making it easier to write and revise longer books. (A different editor of my acquaintance observed that whenever one of her novelists switched to word processing, the average length of their books increased by about 10% .)
There's more in that post including an explanation about why crime and mystery novels haven't similarly bulked up. Do read.
I personally feel this could be a "chicken or egg" kind of a situation here and that there are two trends to be considered in contemporary SF & F:
— the growing acceptance of the literary worth of SF & F and
— the effort that these writers put into creating their fantastic worlds.
SF & F, at least what I know about it, in its early years was much published in magazines and had roots in pulp. This is when the Astounding and other magazines established the short story or the novella as the preferred length for SF & F. Lately, SF & F has become more literary — and writers pay much attention to the characterization, style etc. A reason for this could be that SF & F was easily appropriated by any writers who had a "political agenda" to be conveyed in their writings. Feminist writings are a case in point. As the novels and the concerns (as opposed to the earlier stories which primarily focused on adventure) became more literary the size of the books started increasing. SF & F moved away from its pulp roots and became more acceptable and thus it was alright to publish more of it. Of course, it could be that the move away from standard grocery store wire rack led to bigger books which led to a more literary style, etc in SF & F. I suspect it is a bit of both.
Compared to the earlier days, I have found that SF & F writers today (and of the past couple of decades — the time period Ross is talking about) are more willing to take the time to build their world and characters and slowly immerse their readers in this universe that they are creating. It is almost as if the writers think and know that they are creating something so fantastic and imaginative that their readers could do with some extra help in the detailing of their universes in order to anchor themselves in the story better. This naturally leads to bigger and fatter books. A bigger book allows for more canvas, so to speak, to detail out the universe — which is of such prime importance in SF & F.