There are a couple of important points to take away from the data: (1) France has Europe's largest Muslim population only if you do not consider Russia to be part of Europe--Russia has a vastly larger Muslim population than France, driven especially by immigration from predominantly Muslim central Asian former Republics of the Soviet Union like Tajikistan, and these immigrants are the principal reason why Russia is not depopulating at a rapid pace; and (2) Muslims account for about 7.5% of France's population--well below the 14% of Americans who are Black or the 17% who are Hispanic, although well above the estimates that about 1% of people in the U.S. are Muslim.
So, by the broadest definition of Europe, France has the second largest Muslim population, and it also has the second highest percent Muslim, well behind Albania, which is estimated by Pew Research to be 83% Muslim. Keep in mind that Albania has a fertility level lower than France's so being predominantly Muslim does not mean that fertility will automatically be high. And, thinking of birth rates in France, Ross Douthat has an Op-Ed in today's NYTimes in which he discusses French demographics and claims that:
Demography, the source of so much Gallic anxiety in the past, suddenly has turned in France’s favor: The Germans are rich but aging, whereas even amid economic drift the French birthrate has risen sharply (suggesting a certain optimism amid the ennui). By the 2050s, under some scenarios, France could once again have the larger economy and population — making it either dominant in a more integrated Europe, or the most important power on a continent more divided than today.Talk about poor research!! There is absolutely no evidence that the birth rate has risen recently in France--and certainly not "sharply"--and if you follow his "population" link you will go to a story by the Telegraph in the UK in 2005 for which there is neither support for Douthat's thesis, nor any corroboration by ISEE, the official statistical agency in France. These are the kinds of stories that muddy the public perception of what's going on in the world.