Most Populous Countries in 1800

Very interesting list, indeed. It appears to me, however, that there is some double counting involved - some territories are listed as parts of the Holy Roman Empire and also as parts of some other country.

Heroic guessers. In the Talk page, one of those heroic guessers puts the population of the entire Indian subcontinent at less than 150 million. http://www.populstat.info/Asia/indiac.htm asserts that that is far too low.

Ireland Had 5 Million People Back Then so you should add it

There is, the Holy Roman Empire was not really Holy, definitely not Roman and not an empire, really.

After the Roman Empire spent 1500 years ruling over pretty much all of Europe and the known Western world, and representing everything the West knew about civilization and governance... for a very long time almost everyone claimed to be successors to the Romans. That's why you've got tsars/czars in Russia, kaisers in Germany, kaysers in Ottoman Turkey, etc. And why the Frankish kings thought it important to establish their legitimacy by making their own claim on that legacy (and being crowned by the Pope, thus their holiness).

The geographic area that contemporary humans conceive of as "China" is a huge and diverse place full of several different fertile river valleys that have been teeming with people for a very long time. However, civilization begin much earlier in the West (in Mesopotamia, the Fertile Crescent, and Egypt). Large population centers (cities) arose there first, and several large land empires came to be in the history of Western Civilization that at the time certainly eclipsed any kingdom or dynasty of China during the same period. Notably, the Achaemenid Empire and Sassanid Empire (both Persian, though the latter was a partially Hellenistic empire, too) are estimated to have contained 44% and 37% of the entire world's population, at their greatest extent, respectively. The Roman Empire in the 2nd century contained about 36% of the world's population. And the British Empire in 1900 ruled over approximately 420 million people, compared to Qing China's 415.

Additionally, there was no China when Egypt's Old Kingdom reached a population perhaps as high as 2 million people. The Xia Dynasty, which historians don't even agree existed, didn't begin until around 2100 BC, if at all. The Egyptian Old Kingdom is older than that. The Shang Dynasty began in China 500 years later.

Most crowded, are you serious??? Currently it is at 61th place in population density. So a better question is, was there a period that it WAS the most crowded one, the place is huge. Crowded is the amount of people per area btw, not the amount of people in total, if you meant that you should have used populous. Which since it is rather big, is not that weird there are many people.

Bangladesh, Taiwan, south-korea, rwanda and the netherlands and india are wayyyy more crowded than china. (bangladesh is ridiculously high, above 1150 (pop/km²), the others are approx 400-650 while china does not even reach 150. China is sort of between greece and switzerland when it comes to crowdedness (population density).

Now

I can totally understand not accepting Italy for Kingdom of the Two Sicilies, because it's not the same thing at all, but I wish the quiz did because I'd have only missed Burma.

That's all thanks to the Industrial Revolution. In the couple centuries prior to this, Europe had actually fallen pretty far behind the Far East for the first time in history. Then... the Industrial Revolution (1760-1840)... and they rocketed ahead.

The HRE disbanded itself to prevent Napoleon from taking control of it.

The Habsburgs controlled the emperorship of the HRE but also had their own direct possessions outside the HRE. The wikipedia article linked in the description is poor because it counts many areas more than once for this reason.

Prussia was complicated. The Hohenzollerns controlled areas within the HRE such as Brandenberg but the area governed by the King in Prussia (also a Hohenzollern) was outside the HRE. Similarly, George III was the King of England in 1800 but was also an elector in the HRE (Hanover). England was obviously not in the HRE, just like Prussia also wasn't.