I looked it up, and it turns out that France already had a lower fertility rate than Germany by 1800 (due to factors such as early contraception, France has had a complicated relationship with the Church for some time),but without the land reform laws I don't think it'd have dropped as much in the 19th Century.
After the French Revolution, primogeniture was abolished in France in favor of equal inheritance, which meant that the rural population had a disincentive against having multiple children, for fear of splitting their farms. (France had a particularly high rural population already.) Primogeniture was kept in Britain, Germany, and many other countries that saw greater population growth, throughout most of the 19th century. This is the most believable reason for why France's population growth stagnated that I have encountered so far.Eh, the truth is we don't really know why France had her demographic transition so early. My pet theory is that France had simply less room to grow compared to the other European nations: France had been Europe's most fertile land for much of Europe's history, so when advances in agriculture allowed marginal lands to get high yields France didn't have as much to gain as Germany or Britain.
That doesn't explain why today's France has such an anomalously low population density though.