Afaik in Bavaria and other southern german states had the same inheritance laws as in the code Napoleon. And thats the reason why farms in this area were smaller then farms in northern or eastern Germany. With no effect on the demographics.
Good point, yet there actually was an effect on demographics: those lands were the source of many of the German emmigrants.
So either there are two ways of coping with these inheritance laws: the French way of smaller families (rather difficult IMHO in a catholic country with the general problems of keeping families small in pre-industrial societies) or the Southern-German way of keeping large families of which parts emmigrate.
The other possibility, which I deem more valid, is that inheritance laws are not the cause for the slower French population growth.
In the other discussion mentioned above, I think one major idea was that a higher standard of living in France led to smaller families - a similar effect can still be observed today in emerging countries.
Considering the long-term effects of a higher French population: If there are as many Frenchmen as Germans, WWI as we know it is butterflied away. As mentioned above, neither side will have a sizeable fleet. Both sides will be strong economic competitors for Britain. Therefore Britain will likely keep its splendid isolation a lot longer.
On the other side no side will be able to become european hegemon over the other side - as long as the Austrians are separated from the Germans, which ITTL should make France and AH close Allies?