Here is a graphic comparing the UK, Germany, Italy, Spain and France between 1800 and 2000 to see the difference in increase of population with the current borders:
View attachment 602808
France was ahead the rest of Europe in terms of reduced birthrates, with it being the only country to see that trend in the 19th century, meaning it was largely the lowered mortality and immigration which stood for the population increase during that century. Thus it is not at all a post post-WW1 phenomenon, the French exception is precisely it happening a long time before WW1. To the contrary of the United Kingdom and Germany the French population did not boom, even if it increased throughout the period. France also stayed much more rural than the very urbanised UK.
Here is France alone (I don't know how it factors in border changes):
View attachment 602810