(I meant "contemporary" as in "contemporary with the Commonwealth", rather than "modern".)
That seems very eccentric. Why did the Commonwealth categorise its settlements on the basis of head-count,
Those "contemporary with the 15th century (1400s)" sources, do not mention numbers of inhabitants of course.
Poland did not categorize on the basis of head-count. But demographic studies prove, that towns qualified as "civitates principales" had - on average - much higher population than cities qualified as "civitates secundi ordinis", and so on ("civitates secundi ordinis" had on average more people than oppida, etc.).
Sources say, which cities were qualified as "civitates principales" (in case of Greater Poland region, such city in the 15th century was Poznań

.
Sources also say, which cities were qualified as "civitates secundi ordinis" (in the 15th century in Greater Poland region those were, for example, Pyzdry, Kalisz, Kościan, Wschowa, Piotrków, Łęczyca, Warta, Gniezno, Kłodawa, Śrem, Koło, Konin, Stawiszyn, Pobiedziska, Sieradz, Brześć Kujawski).
Then expert scholars specializing in historical demography, compare how big was average population of cities and towns of each type.
==========================================
Classification of 196 towns and cities from Greater Poland region according to a document from year 1458:
1st category (civitates principales) - 1
2nd category (civitates secundi ordinis) - 38
3rd category - 77
4th category - 80
And here a map showing locations of 39 civitates of 1st and 2nd category mentioned by this document:
The 2 ones which are very close to each other (almost totally overlapping on map of this scale) are Uniejów and Brzeziny:
There was no such state called Polish-Lithuanian Commonwealth (Rzeczpospolita Obojga Narodów) before 1569 - the Union of Lublin.
Before 1569, there were two separate states - Kingdom of Poland and Grand Duchy of Lithuania.
Those two separate states were only connected by a personal union (monarchs from the same dynasty) since 1385 - the Union of Krewo.