Recently Damien Walter wrote about the tyranny of fantasy serial mega-novels. He suggested that fantasy novels now tend toward the enormous because of market forces — because everybody in the publishing and television industries is looking for the next Game of Thrones, and a new author who can open a factory of imagination that will lead to commercial success. I don’t think that’s the reason behind the size or format of fantasy books at all.

Last year, I was teaching on a short fiction course and agreed with the convenor that I’d do genre fiction while he covered high literary, New Yorker-style stuff. But although I read truckloads of fantasy, and write it, it was very difficult to find fantasy short stories that don’t lean in some way on an existing corpus of novels. There’s a very good reason for that, and it’s nothing to do with market forces – and everything to do with the requirements of the genre itself.

High fantasy of the George RR Martin kind hinges on world-building. When there really is a whole world to build, and not just a historical period or a particular country, world-building does not take a few paragraphs in a short story; it takes chapters. Add to that the anvil on which creative writing schools hammer their students now, show don’t tell, and these details take even longer to convey. A very fine example of this is Robin Hobb’s Farseer series. The plot is simple. It’s about a prophet who wants to change the world by bringing back dragons. But each book is more than 600 pages long, and it’s not pointless rambling allowed by an editor who simply wants to sell three books for £20 per hardback rather than one.

The time is taken up by the meticulous portrayal of a friendship. If it were set in the real world, this portrayal would take far less time and space – it’s two men who’ve known each other since childhood, doing variously awful and accidental things to each other throughout their adult lives. That is the stuff of top-tier literary fiction, of Ian McEwan and novels that last an exquisite 250 pages. It’s possible in literary fiction, though, because stories of people in the real world lean on the reader’s knowledge of the real world. There’s no need to explain what sort of government the UK has, nor modern geography, nor what our preconceptions are of particular fashions, because everybody already knows.

However, Hobb’s stories don’t take place in the real world. The two main characters are not ordinary people who can be sketched and left largely to the imagination of the reader. One of them can see the future and refuses to disclose, for cultural reasons and out of general pig-headedness, whether he’s even a man or a woman. The other is a royal bastard born into political circumstances that deny him an ordinary family and any other truly meaningful friendships beyond that of this wonderful lunatic. They live in a world where there is magic in the air and the stones, and a dragon buried in a glacier. All those things which are not mentioned in literary realism but are important for its context – government, geography, fashion, everything – are equally important in this trilogy, but they are not already understood by the readers. To bring it all to life requires a lot of space, and a huge amount of detail.

If you take out the detail of fantasy and boil it down to the skeleton of its plot, the result is nearly always a lot of unexplained magic. That has a quite a peculiar effect.

Game of Thrones: There was once a girl who raised three dragons and set out to conquer the world.

Lord of the Rings: There was once a hobbit who inherited a ring enchanted by a fallen spirit and destroyed it to save the world.

Harry Potter: There was once a young man who defeated an evil wizard.

The stories immediately begin to sound like fairytales. At its heart, high fantasy is what happens when a fairytale-style plot is sufficiently elaborated upon. The magic is explained and systematised; where a fairytale says, “there was once a witch”, fantasy explores what witches are and where they come from, how they live, the culture of witchcraft. It builds its reasoning from the ground up, brick by brick. It becomes sprawling and densely populated because often it isn’t about a unifying plot, but a common world. Everything else about high fantasy is shared with fairytales – settings, objects, stereotypical characters, stereotypical plots. It’s world-building that separates them – and, therefore, length.

To write short fantasy is very difficult. If the usual big-fantasy detail is taken out and you only sketch a plot, you get a fairytale. If you write real high fantasy in 4,000 words, details and all, it tends to be a snippet, not a story. If it’s something set in a basically real world but with a fantasy element, it’s not fantasy so much as speculative fiction, or alternative history, or a ghost story. That means that there is an incredibly narrow taxonomical window in which short fiction can be recognised as fantasy at all. What we recognise as fantasy is long. Sometimes really long.

The fantasy series, and the mega-novel, may have been encouraged by market forces. But they are, above everything else, the natural format for anything so sprawling as a fantasy universe. There’s rubbish, of course; there’s also some properly good writing, and some of it isn’t by Tolkien or Martin. So I hope they keep coming. As long as possible, please.