Losses by Gender
Research conducted by Russian demographers Andreev, Darskii and Kharkova suggests the Soviet population 1941 through 1945 lost 13.5 million more males than females.
In the Soviet-German War the USSR suffered up to 11 million military deaths which would have been overwhelmingly of males. 1941-1945 65,000 Soviet citizens were executed by civil authorities and a further 1,020,000 people died in prisons, camps and colonies of the gulag. These deaths too would have been overwhelmingly of males. Also about 500,000 Soviet prisoners or war, or Soviet citizens in German service successfully avoided repatriation to the USSR at the end of World War II. Thus 12.6 million, or the great majority of the male to female deficit in Soviet population is accounted for by military deaths, judicial executions, deaths in the gulag and emigration of collaborators and prisoners of war. This may attest to at least a rough validity of estimates presented in this paper.
The unaccounted difference between 12.6 and 13.5 million may mean that males were, somewhat counter-intuitively, slightly overrepresented among civilian deaths as well. Possibly particularly due to being overrepresented among victims of German anti-partisan reprisals and other killing policies. It may also mean the number of 500,000 is an under-estimation of how many more males than females managed to emigrate. It may also mean that 11 million is an underestimation of Soviet military deaths in the Soviet-German War. Indeed a noted Russian scholar, S.N. Mihkhalev, estimates the USSR lost 10.9 million Red Amy and NKVD regulars on the front, to military tribunals or in captivity. This would push the combined military casualties from the Soviet population to 11.4 million. Naturally, it may be the 0.9 million difference is a combination of any of the three factors mentioned.
The scale of migration deficit is possibly the most uncertain of all causes of Soviet population losses in the Soviet-German War. The 2.7 million estimate from Ellman and Maksudov used here is twenty years old and is based on only a very rough and preliminary calculation. Even as such, however, it remains seemingly the most-well supported of all such estimates (most of whom are considerably lower). This paper then uses the seemingly best available, but still rather uncertain figure. Hopefully historians and demographers will continue to take a look at this question and eventually produce a more certain estimate, so that the question of how much of the Soviet population deficit in the war may be attributed to emigration may be answered more reliably.
Population of Newly Annexed Territories
Since the Soviet Union conducted a population census in 1937 and then again in 1939 we have a fairly good idea how large the population of the USSR in its 1939 borders was. The much bigger unknown, however, is how many new citizens were added through Soviet territorial expansion in 1939-40. This means the size of the Soviet population on the eve of the war is not fully certain, which makes estimating its losses all the more difficult. Andreev et al, estimate that through annexations the Soviet population increased by 20.3 million. On the one hand Ellman and Maksudov reckon this is more likely to be an underestimate than an overestimate. Indeed there are other estimates which go up to 23 million. On the other hand there are rival lower estimates as well. S.N. Mihkhalev reckons newly annexed territories were populated by between 17 and 20 million inhabitants.