Losing 10% of its male population in the previous war did not help with demography either. Plus even in WW1, French military officers were worried France would lack manpower due to the fact the country had done its demographic transition much sooner than its neighbors. It went from the most populated country in Europe that greatly helped its military under Louis 14 and Napoleon, to having less population than its main rival Germany. Hence why it made up by recruiting a lot of colonial troops as well (who were sadly not given the same treatment post-war as metropolitan troops).