One explanation for why French demographics were so different from those of England:
"The French exceptionalism of small emigration was attributed to the French people's love of their land; for example, one French author wrote in 1860: 't must be recognized that the French rarely emigrate. The fact is that among the various races of Europe there is none with a greater regard for his native land than the French, with a more instinctive, more inviolable affection for his home, his village, and his country...Only religious or political persecution have led in France to emigration on a scale of any importance.'75 However, the key to the difference is to be found within the distinctive French pattern of economic development in the long nineteenth century, the major feature of which was the survival of a large agrarian sector. On the eve of the French Revolution, the productivity of workers employed in British agriculture was already well above that of the French; in familiar fashion, the more favorable British land-to-labor ratios fostered more capital-intensive agriculture, producing a surplus for urban investment, which in turn increased rural out-migration. But in France, the revolution 'gave the peasantry what they had long wanted--full rights of ownership and freedom from the burden of feudal exactions from all kinds.' 76 Although French agricultural output remained consistently below that of the British, the landless peasantry formed a far smaller proportion of the rural population. The characteristics of the agrarian sector in turn conditioned the pace and pattern of industrial development along different lines. Compared with Britain, the workshop sector in France survived much longer, and industry used much less unskilled labor. Most significantly for the present purpose, the rural exodus was long delayed. *The French could afford to love their land, so long as they kept their families small.* [my emphasis--DT]
"In short, France made the transition from agriculture to industry and from rural to urban life without experiencing the shock of the Great Transformation. Not only did fewer of the French leave France, but fewer also moved to great urban centers, because the push on rural localities was much weaker than elsewhere. If 'migration begets migration,' the reverse is true as well: the absence of emigration in the early period of transition makes it less likely that, should a 'push' subsequently arise, emigration will follow. Concomitantly, in the absence of surplus population, the state had no reason to turn emigrationist. Although the precocious limitation of fertility in the middle decades of the nineteenth century was rational from the economic perspective of the rural population, it was problematic in other respects. Hence, uniquely in Europe, from the middle of the century onward, the French state became decidedly immigrationist..."
http://books.google.com/books?id=Dqawo0Kpvz0C&pg=PA53
For a related but somewhat different analysis:
"It is instructive, in this regard, to review two recent syntheses of what has been called French exceptionalism, the peasant-based fertility decline of France.
"One is by Jean-Claude Chesnais, who begins by citing the revolution of ideas and decline of religious sentiment that unfolded in the course of the eighteenth century, culminating in the political and social revolution of 1789... Dismantling the hereditary estates of the past, this transformation resulted in a more open society in which social ambitions and the thirst for equality were realized. The "pursuit of happiness" thereafter came to include the enhancement of material well-being, perhaps at the cost of having more children.
"Consistent with his interest in migratory movements, Chesnais goes on to cite the failure of France's colonial policy to secure overseas outlets for "excess population," this limitation being reinforced through anti-emigration laws. In addition he analyzes the divergence of French from English agricultural history. Already smallholders before the revolution, French peasants, through their rebellions, interdicted the penetration of a central feature of agrarian capitalism—legally sanctioned enclosures—into the countryside. (Significantly, neither potatoes nor cottage industry gained much of a foothold either [see Lesthaeghe 1990: 18].) This, Chesnais argues, delayed the formation of an industrial proletariat and with it France's industrial revolution, but precipitated a demographic revolution as yet more smallholders remained on the land. The "triumph of the small" or the "revenge of the small against the large estate" was realized. Two-thirds of France's 32 million inhabitants in 1830 belonged to landowning families; in Britain, including Ireland, only 54 percent had this status (see Hobsbawm 1962).
"Chesnais considers, but rejects, the nineteenth-century thesis of Le Play that the Napoleonic Code, with its provision for equal divisions of property among all heirs, was the decisive element inducing peasants to limit family size or face an intolerable fragmentation of their resources. Fertility decline began before the code was instituted--indeed in some regions before the revolution. There was even early evidence for it in places of single-heir inheritance in southern France. He concedes, however, that enforced partibility may well have intensified a process already underway..."
https://books.google.com/books?id=Hgzorr7xG3QC&pg=PA200
So you really would have to prevent the Revolution and the advantages it gave to the small landowner--and even that might not be enough, given the divergences that already existed between French and English agriculture before 1789.