Parts of one of the different sub-settings within Behind Gently Smiling Jaws involve areas which appear small on the map but which are very large. So a chamber which appears to be 100 yards across on the dungeon map could actually be 100 miles across. If the PCs move around the walls the chamber has its "ordinary" map-based circumference. But if the PCs move across it, it might take them hours or days to do so, and it might contain entire countries.
I have been thinking about this off and on for a while - not just for this game but also because I nurture ambitions of one day running a Mythago Wood campaign. (The eponymous wood looks like an ordinary smallish English wood but is actually vast - potentially infinitely big - once you get inside it.) It would look a bit like this:
The players mosey on down the corridor in the ordinary way. If they move around the walls of the chamber they find its circumference what it appears to be on the map (say, 300 yards-ish, because the chamber looks about 100 yards in diameter on the map's scale) but if they were to move across it they would be in, basically, a hexmap.
I feel as though this ought to be more difficult than it appears. The main thing to keep track of is just what hex the players enter. (The hexes would need to be numbered in reality of course.) They would then move around the hexes as though in a hex map, say on a 1-mile per hex basis. Once they got to the edge of the hex map, they would find the cavern wall wherever they emerged.
FLY, MY PRETTIES. Tell me whether there is anything about this concept that would make things particularly difficult to use in actual play.