I think if you increase the population sizes of settlements you run into issues though with the tone of the game. Also the old books listed distances. The old books included maps of towns with surrounding farmland. And the cultural levels were pretty all over the map. My point is, the settlements they list, are, I believe just meant to be notable settlements. I think countryside populations, and other settlements are assumed (this is one reason you have the homestead encounter in Feast of Goblyns). But it is meant to be a bleaker place and it is not the real world place So as much as I love Harn style thinking about population density, probably isn't as important to making Ravenloft make sense.
And again, many of the major settlements are fairly small in Ravenloft. Many of them come across as backwaters. Il Aluk was one of the bigger cities in Ravenloft with a size of like 25,000 which feels right to me (and while it isn't 100,000+ size city, keep in mind that not much smaller than Athens during the renaissance). For the vibe. I don't know. I love stuff like Harn, I actually quite like real world demographics, but for Ravenloft, I think you want the places to fit the tone of the source material.
Also there is some amount of flexibility when reading settlement population sizes in a game. Do we assume that is only the population that lives inside the city (or does that number also include the people living around the city). How you answer that question can easily double or triple the size of the population. Ultimately the setting is barebones in its presentation to allow for a wide range of interpretation. If you assume more population surrounding the settlements, assume more settlements than the ones presented on the map (which I do, and I always saw that as default), you get pretty close to that x10 number. I just personally wouldn't do it by increasing city sizes themselves, and I think it is the kind of thing that can vary from campaign to campaign