Here’s a second draft, with the material on overpopulation added. It is brief enough that by sweating fifteen words out of the section on “Habitable Planets and Moons” I was able to squeeze the two into a single two-page spread.
Disregarding brand new worlds whose first pioneers landed only a few weeks ago, the least populous world in Flat Black is Lowrie, a pleasant planet in the outermost Periphery. Lowrie received no settlers to follow its terraforming crew, and is now occupied by 9.9 million nomads and poor farmers at a population density of 0.044 people/km².
Worlds nearer to Earth were settled sooner, and received more immigrants. The oldest colony is Tau Ceti, which was settled 850 years ago and received 3.9 million immigrants over 250 years. Tau Ceti now has a population of 5.9 billion at a density of 97 people/km².
But seniority is not the only factor. The most populous world of all is Margulis, 89.5 light-years from Sol in the inner Periphery. Margulis was settled 630 years ago, receiving a total of 57 thousand migrants. But it has vast lands, coped well with the sudden loss of Earth, and has social features that encouraged large families. Margulis’ population is 21.5 billion and its population density 71 people/km².
Of the 68 worlds in the Core, the average population is 2.34 billion and the median 1.32 billion. 42 worlds have more than a billion population and three have more than 10 billion (Tian Longshan, Lahar, New Athens).
Of the 557 worlds in the Periphery, the average population is 1.1 billion and the median 453 million. 170 worlds have over a billion people and two have over 10 billion (Ursula and Margulis). 62 worlds in the Periphery have fewer people than the least populous world in the Core (Lahar; 74.4 million).
Meanwhile, the Fringe is a gigantic dark horse. Without making much impression on minds in the old worlds, the Empire has developed 375 worlds and sold land and passage to 93 billion migrants. The most populous new world is Florida, which has 2.65 billion residents. 26 new worlds have populations of more than one billion.
Crowding and overpopulation
No planet or moon is anywhere near being covered with a cityscape. The most densely-settled is Iter, which has 132 people/km² of land. That’s more that 2.5 times Earth in the early 21st century, but less than one eighth of suburban sprawl. Iter has in fact vast high-rise conurbations, which stretch as far as the eye can see even from the top of a 100-storey tower, but between them it has great expanses of high-biotech farms, and between those tracts of desert, mountains, and ocean.
As for very low population densities, on new worlds and marginally habitable planets they usually represent small patches of settlement in the midst of unoccupied, perhaps infertile, land. But on worlds that are salubrious but economically backward they reflect dispersed, rural settlement structures.
Dry worlds, hot or cold worlds, dimly-lit worlds, or worlds where the terraforming is superficial or incomplete often have low carrying capacity, or may be fragile, They may be overpopulated even at low population density. Over half the worlds in the Core and inner Periphery suffer from scarcities or environments degradation because their populations exceed their sustainable carrying capacity with the tech they are using.