Hi. So I changed the cap before generating a world, to 8228 and the site cap to 777. I also doubled megabeasts and all "villain" types, werewolves, night creatures, etcetera. Though I did remove bogeymen and evil rain, and all the "disturbances" I doubled from 52 to 104. As for titans, I increased it from 33 to 321. The populations for my worlds are always really low for the "nice" civs, in fact there is one elf, 7 humans, and 109 dwarves in this recent world. And the goblins are 50,000! Even more than that, I just rounded. Why are goblins allowed to break the population cap?
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OK. I think I chose the right numbers. The pop cap this time was 11111 and the site cap was 1111. I find the original values pretty extreme, as the sites clutter literally entire rivers and mountains. This left alone plenty of nice natural areas and all civ populations are averaged around 8000 - though elves are a bit lower than the others - instead of one making all the others extinct, poor kobolds.
The numbers for goblins can rise massively from the standard disturbances and villains. Halving the numbers puts goblin numbers at equal generally, with other civs, whereas even doubling them, will multiply the goblins six-fold! So, I've put all "interaction" and "villains" to 13, which is either half or a quarter of the original numbers, removed bogeymen and evil rain, and kept my megabeasts at double the original and titans at 321. I really like pairs of dragons and cyclops, and such.