My colleague Jake shared this with me today: "The article I mentioned to you is: Getty, Rittersporn, and Zemskov, "Victims in the Soviet Penal System in the Pre-War Years: A First Approach on the Basis of Archival Evidence", American Historical Review, v. 98, n. 4, Oct. 1993. As the title suggests, it uses archival data, not available until the early 1990s, to determine the nature and extent of the Stalin era prison system. Using data from the article and population statistics for the USSR, one can see that in 1939 - at the height of Stalin's power - the prison population amounted to 2.4% of the adult population. By contrast, the prison population in the US today amounts to 2.9% of the adult population."