Because France is an outlier with regard to the demographic transition model, in two ways:

• Firstly because the fall of mortality that initiates the transition began in the 18th century, earlier than in neighbouring countries (c. 1760, which makes France the first country in the world to experiment this change). In 1789, France was Europe's most populated country (which is the reason why it was able to play such a role between 1789 and 1815: there was simply no country in Europe was able to put on the field so many armies by the mean of a levy).

• More importantly because the time between the fall in mortality and the fall in natality (the transition in itself) was shorter than in other European countries. In fact in some places the fall in natality arguably predated that of mortality, something quite bewildering, and a great variety of explanations have been given. It seems that contraception was much more efficient in France than elsewhere; some people have argued that the Napoleonic campaigns (and the frequentation of prostitutes by soldiers) had helped bringing to the populations a greater variety of techniques, which may true, but does not account to the changes of the late 18th century (in fact, some historians suggest that in the upper class, simple contraception methods like coitus interruptus were already popular before that time, and that the fall of natality is a result of their diffusion). A simpler reason may be that France had always been quite dense, and that malthusian practices developped because of that. Some people like Daniel Scott have also argued that, not unlike America, it was due to a greater emancipation of women, who adopted birth control methods. No definitive solution of this problem has been found.

French were the first to develop birth control practices throughout their society. The "pull out" method and douching was common 100 years before it became common outside of France.

Source: Children and Childhood in Western Society since 1500 by Hugh Cunningham.

This is actually a super interesting question that came up when I studied Vichy. This may be a bit specific in terms of context, but bear with me.

One of the main platforms of Vichy's Révolution nationale during World War II was to restore moral order to France. Moral order was heavily linked to what Robert Paxton describes as the "solidarity and fecundity of the French family." France's declining birthrate was seen by Vichy as a failure of Republican moral order, a symbol of national decadence, and one of many reasons for France's uneventful fall to Germany.

Vichy was not the starting point for French population growth efforts. Parliament had outlawed birth control in 1920, for example.

Much of Vichy's efforts, however, involved undoing one of the principal causal factors for France's declining birthrate—Napoleonic inheritance rules. Over a century previously, the Napoleonic Code had abolished the old system of primogeniture, where the eldest male acquired the entire family inheritance. Instead, Napoleonic inheritance stipulated that each child receive an equal inheritance as all other children in the family. The problem was that this created substantial disincentives to having more children; parents, wanting to do well by their children, could not have larger families without being forced to divide their inheritance into ever-smaller portions. Imagine a family farm being divided into increasingly-unviable sizes with every passing generation and you can get a sense of why families were hesitant to reproduce. Having fewer children was the only way to prevent family inheritances from being broken up to the point of counting as almost nothing.

In response, Vichy took steps to mitigate the Napoleonic inheritance concept. For example, Daladier's 1939 Family Code "favored resettlement of farms by providing loans to young settlers who interest and payments decreased with each successive child; it had breached the Napoleonic concept of equal inheritance by providing that the son who stays home and works a farm inherits a greater share of it than his city brothers and sisters." (This also demonstrates Vichy's rural slant).

Vichy also demonstrated favoritism towards large families in other ways. For example, government jobs often emphasized paternity to the point where men with many children were granted high-ranking seats on government committees, whereas childless men were granted undesirable posts.

All of this fit within Vichy's narrative of the importance of family superseding the rights of the individual. The glorification of the family as the organic unit of French life; the moral crusade against divorce, prostitution, and other vices; the restriction of abortion—all served as key markers of Vichy's Révolution nationale.

As Paxton points out, the War was a strange time to raise the birthrate, to say the least. But Vichy was at least somewhat successful; by the end of the war, the French birthrate reached heights it had not seen for a century.

Source: Robert Paxton, Vichy France: Old Guard and New Order.

*Edit: Daladier's 1939 Family Code was obviously pre-Vichy. However, Vichy would continue to propagate and supplement this particular policy during the War. Excuse the brain fart on my part.

Was this lower French population growth true in New France? I'd always been under the impression that Quebecois fertility rates were higher than normal, as shown here, is that a relatively recent trend? (A trend that has reversed since about the 1960s)