Rule 20 of 24 · Chapter V — The Unseen Mass
Imply the vastness, don't catalogue it
Why this rule exists
A world feels large when it is suggested and small when it is inventoried, because a list, however long, is finite and closed, while an implication opens onto the infinite. Name a single far country in passing, mention a war the reader will never see, refer to a custom without explaining it, and the reader's mind expands the world outward past the edges of the page. Try instead to catalogue your world's every kingdom, god, and species, and you paradoxically shrink it, converting a living hinterland into a bounded encyclopedia the reader can see the end of. The horizon that recedes is vast; the horizon you have carefully drawn on a map is merely large, and only as large as the paper.
In practice
Gesture at more than you describe, letting names, references, and fragments imply a world that continues past the frame. Drop mentions of places, events, and peoples the story never fully visits, and resist the urge to follow up on each with an explanation, since the unexplained reference is the one that expands the world. Prefer the glimpsed to the surveyed: a caravan from somewhere you never name, a festival for a god you never describe, a scar from a war the story does not tell. When you feel the itch to catalogue, to list all the houses, all the realms, all the magics, indulge it in your private notes and show the reader only the tip.
When it doesn't apply
Certain forms, the sprawling epic, the deliberately encyclopedic novel, make abundance and catalogue their pleasure, and readers come to them for exactly that plenitude. Some fantasy delights in the exhaustive gazetteer. The rule is a default for creating the feel of vastness efficiently, not a ban on the maximalist mode for writers and readers who love it.