Adding up the 1780 numbers on that page, the numbers appear to be almost identical. But there's a catch: those numbers include enslaved people. who numbered at least 500,000 (see [1]).

* Free states: 1,390,067

* Slave states: 1,390,302 - 500,000 = 890,302

> Still not that massive in relative terms

I don't know why you persist in saying this.

[1] https://userpages.umbc.edu/~bouton/History407/SlaveStats.htm

Persist in what? My original was that population sizes between states were relatively more even back then than now.

1780 is probably not the best year, though. e.g. New York was still a slave state. Of course the gap only grew bigger over time