I'd read many times that during World War One, when France made On ne passe pas! a national slogan, the French population was seriously outnumbered by the German one, but I hadn't realized how startlingly different the population trends of the two countries are. When I put the following chart together, I was floored.
When France conquered most of Europe under Napoleon, its population was nearly 50% larger than that of the territory that would become Germany. In 1871, at the time of the Franco-Prussian war (in the wake of which the unified German Empire was declared and France lost Alsace-Lorraine) the two countries were evenly matched in population. During the forty years between the Franco-Prussian war and the Great War, the German population increased by 50%, from 41 million to 65 million, while the French population increased by only 10%, from 37 million to 41 million.
During the 20 years between the wars, despite the heavy military casualties of the war and the civilian casualties resulting from the British naval blockade and the Spanish Flu epidemic of 1918-1920, the German population grew by 7 million while the French population grew by only 2 million from its post-war trough.
Given all this, the other fascinating thing is that the French population then took off and began growing again and has done so steadily since World War II.