As u/sataniel98 said the population of the core of France is much lower, about 27 million people.
As for your other question you might find this other answer interesting: https://www.reddit.com/r/AskHistorians/comments/1wn9qr/why_did_the_french_population_grow_much_less_than/?utm_medium=android_app&utm_source=share
And this answer has a number of links to other similar questions that could answer it more in depth: https://www.reddit.com/r/AskHistorians/comments/1mwlvq/why_did_frances_population_stagnate_in_the_1800s/?utm_medium=android_app&utm_source=share
Check the great answer by /u/etan-tan, they brought most of the point I would have want to talk about, there is just one idea I think I can add however. For context I have a licence degree in history from France and took a great class on the religious anthropology of France from the Revolution to the council of Vatican 2.
What I want to outline is the corrolation between three maps, the maps of priests who refused to signe the national convention of the clergy during the Revolution, the map "Boulard" which represent the result of the years long investigation by the Catholic church on the state of religious practice in 1947 and finally the map of the current rate of children by women in 2003-2007. I would have prefere to include a map of the rate of children by women during the 19th century but I can't find it in my files. Religious antrhoplogic study have, since the 60s, studied the correlation between the area where most priest refused the convention, chiefly due to it's requirement to be loyal to the State first, and the area that the 1947 marked as still very heavily practicing. Those area can be packed in four zone : Brittany, Alsace, the north (Pas-de-Calais) and the center. There is also a correlation with the birth rate map that shows those area tend to have a higher natality rate. This tend to lend credance to the idea of a cululral factor in the comparable low birth rate in France compared to other country. Now, I am very carefull with this because it is very much in debate within the accademical community. And it is by any means the sole or major explenation, but it has gathered more and more interest in the last two decade as a possible subject for further research.
The map of conventional priest where lighter shade indicate higher rate of refracarian priest
