At the time of the Franco-Prussian War the populations of France and what is now Germany were roughly equal, yet by the beginning of World War One France had fallen behind Germany and even Britain. Aside from territorial additions adding to the German Empire’s total, why did France see the larger demographic flattening, and not Britain which was first to industrialize?
Part of the answer is that France's demographic decline began well before industrialization, for reasons unrelated to industrialization — though when it did industrialize in the 19th Century that doubtless played a role. Among the other factors causing France's demographic slowdown were casualties from the Revolutionary and Napoleonic wars, inheritance laws that encouraged poor farmers to have fewer children, and the collapse in authority of the Catholic Church, which then and now opposed contraception. Two of my previous answers go into this topic in more detail, though this shouldn't preclude additional answers: