Neither the loss was that devastating nor the period between the wars was that short. We are speaking about 21 years between the 1918 and 1939, so the entire generation that has not yet been born when the Germany signed their capitulation in Compiegne could have been conscripted in the year when the Second World War broke out. We need to remember, that people die during the wars but they are also getting born, especially when the civilian population is relatively safe, what was more or less the case in the First World War due to limited forms of warfare available. This is evidenced by the demographics of Germany: in 1914 it had 65.8 million citizens and in 1919 this number fell to 62.8 million, but the overall population decreased only in the last two years of war. What is interesting, the fertility rate and the population balance started to significantly decrease after the war (in the first 14 years of the century it was roughly 13 per 1000, then fell to 8/1000 in the 1920s and then to 6.5 in 1930s*). Despite that, with still high number of births, Germany was able to replenish its manpower, so to speak and was able to reach pre-war number of citizens (65.8 million) in 1932. In 1939 it reached 69 million, over 3 million more than Germany had in 1914.

Please note that this recovery was matched by France that went from 41.6 million in 1914 to 38.6 in 1919 and then back to 41.5 in 1931 but then remained roughly constant until 1939. It should have been noted though that despite similar crude birth rate, the net population change was very small, in the ballpark of 2.5/1000 with several years in the interwar period registering negative change due to high death rate. Last but not least, after Anschluss in 1938, Germany was able tap the human resources of Austria, inhabited by 6.7 million people at the time, effectively increasing their conscription base by 10%. Thus, Third Reich with its 76 million of inhabitants in Germany and Austria was almost doubling the 41.5 millions in France.

*Just for comparison, this factor in the last 19 years averages to -1.8/1000 (note the negative sign).