In the centuries post-Breaking, humanity rebuilt itself and reach a kind of new pinnacle that, while nowhere near the AOL, was vastly above the NE conditions (fast travel through the ways etc., still powerful AS that were not as cloistered etc.).

The Trolloc Wars decimated humanity to a degree we can't be certain of, but at least to the point the "golden age" of the Ten Nations could not be ressurected. Right before Hawkwing's rise, a major plague decimated a lot of the population, and a few decades later there was the War of Hundred Years, which no doubt diminished massively the male population. Nation arose after the War, probably centered on the provinces of Hawkwing's empire, but quite a few did not survive. One factor is that Hawkwing in his efforts at control and pacification forced mass migrations during his reign, forcing people from one nation to move to other province, shifting the nobility around a lot too. If we look at the maps, it seems that most of the nations that didn't survive in the New Era were located in between the central nations and the north. We know two important things that may well explain this: Ishara's nation (Andor) rapidly stabilized and became one of the new powers on the continent; the borderlands also stabilized fast, the threat of the Blight sparing them from the worst of the WHY. Ingtar provided another information: the nations south of Shienar ressented the fact Shienar "abandonned" them. We also know that western Andor was the result of a later expansion, in the reigns of the Queens following Ishara.

The picture it paints is that there was a either a vacuum or an unstable territory south of Caralain, with the additional pressure of a major trade and military power centered on Caemlyn-Aringill, with yet additional pressure from the very stable Borderland nations. Why did the nations between the Borderlands and Andor fell apart? It appears the answer is that early stability and arising power caused emigration to those nations, which along centuries eventually caused the collapse of those nations.

The nation between Tear and Illian very possibly collapsed because the center (Far Madding) became a major trade center (attracting people), while the constant wars between Illian and Tear eventually brought the migration of the countryside population in this area to one of the three major cities (FM, Tear, Illian).

The wars between Cairhien and Tear quite possibly had the same result for the nation standing between those two: peasants eventually got out of the way of the two major powers, tired of starving because they fed out armies on the move living out of the country and harassing (rapes etc.) them.

As for Almoth Plain, the collapse seems more economic in nature: it was caught between three major trading nations (Andor, Arad Doman and Tarabon) and didn't manage to compete nor protect the trade routes (the passes in the mountains near Katar are a major assets, as it provides access to the north-east to Arad Doman and Tarabon, and access to the west to Andor, probably causing a lot of competitions between the big threes for influence over this area. More recently, it's been mostly AD and Tarabon which fought over control of the area and its trade routes, Andor has pretty much lost control of its western part).

We don't know for sure if and how much the numbers of citizens had declined in the New Era, but the power of the WT (in the political as well as knowledge-skills senses) and the presence of AS in "normal life" has declined a lot since Hawkwing's days, meaning more wars, more diseases etc. We actually don't know if the "major cities aren't bigger than ever now, while in the days of Hawking, there weren't more cities, but of of a smaller size. Caemlyn, Ebou Dar etc. have expanded a lot, meaning humanity gave up a lot of territory, because the populations tended to seek the safety and better economical conditions of the cities. It's well to remember that city dwellers always have far less children than peasants do, as for peasants children are a work force, while for citizens too many children are a burden and result in pauperisation over time. Living conditions in cities aren't as good, resulting in a higher mortality rate. If the countryside population tended to migrate to city (as we've seen some of that recently, eg: in Cairhien after the Aiel invasion when the peasants moved to cities and starved there...), this would result in a net decline in population though the next generations, as family that used to have 6+ children now tended to have only 2-3 (surviving, anyway).

A nation like Shienar probably shrunk after the collapse of Malkier, when it "became" a Borderland nation and a great deal of its population had to move around fortress-cities for protection. The peasantry has now smaller farms, so don't need as much of a workforce, and city-fortresses would support only the peasantry it can protect and that can sustain the city. These are all incentives toward a lower natality rate.

The trading networks have also declined for all sorts of reasons (distances between trading centers are much longer aftet the collapse of many nations, for one). Less people in the Borderlands mean a higher percentage of males are forced into a military life, which results first in a larger percentage of men making a para-military career (weapon smiths, iron miners and so on), and then in a smaller percentage of men available for heavy-duty jobs like lumbering, mining etc. which is then compounded by the fact most of the population is forced to live around city-fortresses because of Shadowspawn raids, meaning too high risks areas with ressources are given up. This all results in much smaller possibiliies for trade and agriculture in the Borderlands than in the "old days", and that means nowadays the Borderlands pretty much sustain the population to guard the Blightborder and those needed to sustain their needs. Kandor is lucky, because it has a high value defendable (it's located south of the cities and fortresses) resource in diamonds-precious stones. We haven't heard much about trading goods from Arafel, and Shienar specializes in lumber.