This isn't a new idea. But, in recent times I've become fond of the idea of a fantasy setting with a small population, like in the Elder Scrolls Series, where a metropolis (which in reality would need a population in like the hundreds of thousands) has a population of around 313 (I'm using the Imperial City in Oblivion as a representation of a metropolis).
The advantage/goal of this is to reduce things down until you could literally write up lore/background/etc on each and every NPC in the entire setting
(or at least a good portion of them; maybe all the important people and then some random unimportant people for each area).
I'm thinking of like,
log(x)^2 = y
where X is the real-world population and y is this reduced population.
The disadvantage to this (in the Elder Scrolls at least) is that you have to throw common sense out the window. GURPS Space states that any population below 10,000 would be unstable
. I'm not sure of the exact reason for that, is it just genetic diversity? (cause I can totally ignore genetics if that's the answer). People dying of unnatural (accidental) causes don't seem like they would be enough to really factor into this 10,000 limit, barring large scale events like plagues.
To match up with the Elder Scrolls we'd have to reduce distances down as well (being able to cross an entire kingdom in one day's walking isn't realistic).
I think the reason for cities being distant from each other has to to with taxing the resources of the land. Aquifers can dry up if you use them too much, game animal populations can be hunted to extinction, only so many crops/plants can grow in an area. Would need to tweak these things to ensure that multiple communities are created (don't want the whole world grouping up in one big city). Might even add new resource limitations, like a reduced quantity of air, so that if too many creatures gather into a small area they start running low on breathable air.
One way to help reduce distances would be to reduce the diameter of the planet, which would also help limit the population. Though, if taken too far, the repercussions on the horizon line/line of sight would be weird.
Reproduction rates are an issue in this setting, increasing gestation periods could help create this sort of situation, maybe, and/or reducing the chance of pregnancy.
One thing I'm thinking of to help make this work, is to have an entirely sterile planet, or rather, a planet where pregnancy just doesn't exist at all. People could be born via magic ceremonies, or be given as gifts from the gods, or, something... Needs work. Magic ceremonies don't work well since a deer couldn't really perform a ritual to create a fowl. It might work out if like a special race of animal-loving druids were the ones performing the ceremonies, and all creatures are just naturally friendly to that race. Would be interesting if they could choose the nature of the creatures they make; leading to things like people trying to bribe them, evil druids creating evil creatures, etc.
Another possibility is that people could be like pheonixes, each time one dies they burst into flames and another is born from their ashes. It would be interesting if like, death by drowning for example, became a terrifying idea because it would end not only your life but your future "reincarnations". This would put a hard limit on population.
Anyway, I'm having a hard time realizing all the repercussions and problems that a small population would produce, so I'd appreciate any help!
I'd like to build this into a system for people to use when building settings if they want to, but it needs more figuring out.
edit: somebody on the UESP wiki just pointed out this super relevant page
that gives the populations of the various cities in Oblivion