Regarding the Russians and East Slavs more broadly, Anatoly Karlin has some napkin math that at the very least shows the huge toll that the world wars had on their populations, which barely grow or s:
(8a) Russia just within its current borders, assuming otherwise analogous fertility and migration trends, would have had 261.8 million people by 2017 without the triple demographic disasters of Bolshevism, WW2, and the 1990s – that’s double its actual population of 146 million.
(8b) According to my very rough calculations, based on various sources, the population change for each of the following in their current borders between 1913⁄14 and 1945⁄46 was about as follows:
Russia – 91M/97M
Ukraine – 35M/34M
Belarus – 7.5M/7.7M
Assuming a threefold expansion in all of these populations, we could have been looking to a Russian Empire or Republic with a further ~120M fully Russified Belorussians and largely Russified Ukrainians, for a total Slavic population of almost 400M.
That’s twice bigger than the number of White Americans today, the most populous single European ethnicity, and almost as much as all of today’s Western Europe.
(8c) Total population of a hypothetical Russian Empire that also retained Central Asia and the Caucasus, and that hadn’t been bled white by commies, Nazis, and Westernizers during the course of the 20th century, would likely have been not that far off from Dmitry Mendeleev’s 1906 projection of 594 million for 2000.
Regarding the Russians and East Slavs more broadly, Anatoly Karlin has some napkin math that at the very least shows the huge toll that the world wars had on their populations, which barely grow or s: