The area encompassing modern France is thought to have had ~three-four times the population of England and between 1.5-2 times the population of Germany in the middle ages (see https://sourcebooks.fordham.edu/source/pop-in-eur.asp). By World War I, this had changed drastically, with the population of Germany being larger and that of England being around equal in size. This had dramatic effects on the ability of France to go toe to toe with Germany in the Franco-Prussian war and in World War I. Presuming that the early industrialization of England gave it a heads up in population growth, why was there then such a big change in the relative populations of what is now Germany and what is now France? I realize that this is a complex question, but I think it is very interesting and wanted to offer it up for discussion.
I addressed some of this in an earlier answer to a tangentially related question.
u/datedusername is right to highlight the devastation of the Revolutionary and Napoleonic Wars. The one million dead Frenchmen from the wars, as a share of the population, would be equivalent to about 11 million Americans dying today; it's a higher casualty rate for one country than both sides combined suffered during the U.S. Civil War. Historian Robert Tombs estimated that "more than one in three of all [French] boys born between 1790 and 1795 were killed or wounded." (See: Peter McPhee, A Social History of France: 1789-1914. 2nd ed., and Robert Tombs, France 1814-1914)
Another key case, which I talked about in my earlier answer (about why France didn't send more emigrants to the New World in the 19th Century), is a seemingly minor legal ramification of the Revolution. French inheritance law was changed from primogeniture (where the eldest son usually inherits the entire estate) to a principle of equal inheritance where estates are divided up among multiple children. This meant that the more children a French family had, the less able they were to preserve the family farm or business as an intact entity to future generations. That's something many families very much wanted to do, especially in rural areas — and most French people through most of the 19th Century were rural farmers. Given this incentive, they responded by having fewer children:
Given the desire and need to keep small family holdings intact, rural people responded by deliberately limiting family size, usually by coitus interruptus, but also by using knowledge of the fertility cycle, abortion, douching, abstinence and occasionally infanticide. (McPhee)
By the French Revolution, France was the most populous country on the continent, but the Napoleonic Wars had a devastating effect on the nation. Over two decades, somewhere in the neighborhood of one million Frenchmen were killed in the wars out of a total population of 25-30 million people, depending on the year or Boundaries of the country. Germany was also largely insulated from the revolutions of 1848, which in a general sense cause brief but sharp periods of economic and social instability which can effect both economic and population growth, and again suffered fewer (by perhaps three quarters) casualties than the French in the Franco-Prussian war.
The wars of the early 19th century threw France's population balance out of whack and most likely contributed to the low birth rate throughout the century. The claim made in "The Pursuit of Glory: The Five Revolutions that Made Modern Europe" (if you're looking at the sources on Wiki) is that there were .857 males for every female at the end of the Napoleonic era. This was coupled with nearly two decades of several hundred thousand to a few million men being mobilized and away from their farms, their wives, etc.
I don't think this comes near a complete answer as it only really touches on a few demographic reasons why the French population did not grow at the rate seen around Europe. I guess it is important to note that France really is the outlier in Western Europe in the 19th century with its low growth rate, whereas Germany and Britain grew at similar (much faster) rates.
populations in Europe in the early 1800s: Mitchell, B.R. International Historical Statistics: Europe 4th ed. London: Macmillan, 1998. Casualties in Napoleonic wars: https://www.jstor.org/stable/6495?seq=1#page_scan_tab_contents Particularly the Russian invasion: Zamoyski, Adam (2004), Moscow 1812: Napoleon's Fatal March