That's a really good point, chaos 45, I hadn't thought of it in that way. Obviously, I took much fewer losses as the USSR in the first game (though perhaps not in the second). The Red Army enlisted 29,500,000 men during the course of the war in addition to the nearly 5,000,000 who were in service on 22 June 1941. There's 202 turns from 22 June 1941 to 8 May 1945, that works out to an average of about 149,000 a turn.
In game terms, the USSR has 3936 manpower centers at start. I estimate that historically they lost about 600 of those. I don't know what proportion of the manpower of captured cities is saved through evacuation to the east, but in my game that had the most historical 1941, I have 3009 manpower centers in spring 1942. The current manpower production multipliers by year are 50 (1941), 40 (42), 35 (43), 30 (44), and 15 (45). There are 29 turns in 1941, 52 each in 1942-44, and 19 in 1945. So based on 3000 manpower centers, the USSR in game terms will produce 4,350,000 men in 1941, 6,240,000 in 1942, 5,460,000 in 1943, 4,680,000 in 1944, and 855,000 in 1945. Actually, there would probably be some manpower centers rejoining the homeland of world socialism (TM) as the Red Army advances, but we'll take this as a conservative estimate. That's a total of 21,585,000.
Running through the list of reinforcement units, there's about 1,100,000 men in those units, though at least some of them (about 300,000) are transfers from the Siberian front who would have been counted among the original 5,000,000. There's still about 7 million Soviet soldiers who just vanished into thin air.
I suspect an uncharacteristically inefficient intervention by Thanos.