AFAIK there was quite a lot of emigration from Italy and Eastern Europe which is why many French people have Italian and Polish surnames.There are plenty of ways to boost the French population, though to do so significantly is difficult. Acquiring the whole of Belgium is not likely, but that along with A-L would indeed get to 50 million. More than that, though? Birthrates will certainly be a little bit different in a highly industrialised (North) France, but probably not in a positive sense. Fertility campaigns generally do not have huge impacts in developed countries. IMO immigration is the best bet for a significantly higher French population, perhaps encouraged by a more dynamic French industrial economy. Italy and Eastern Europe had loads of catholics to export, all it takes is a demand for labour and a French willingness to allow it. Not easy, but not impossible.
I don't know how, but my understanding is that French government policy in the nineteenth century helped to lower it's birth rate compared to Germany. Can anybody confirm that?
The other possible source of people is the French empire in Africa. AFAIK recruiting Arabs and Africans into the French Army was one of the ways they made up for their lower population.