It's mostly a matter of when the demographic transition started: https://nitter.net/pic/media%2FGEDBLvOXUAANUlK.png%3Fname%3D...

France used to be “the China of Europe” (which is why we kept being at war with the whole continent at once). Had France followed their neighbors' demographic, it would be home to more than 200 million people today.

The demographic collapse of France in the 19th century, while Germany kept growing, alone explains the French defeat in 1870 (and then the two world wars).

More data on that piece of history, and a hypothesis to explain it, here: https://worksinprogress.co/issue/frances-baby-bust/