It did not - the population inhabiting the current French territory had represented 20% of Europe's population quite consistently since medieval times. E.g. France numbered 19.7 million inhabitants as early as 1457 [1]
The population explosion that's discussed in the article (the population dividend of the demographic transition) just never happened in France - which is the whole point of why its relative standing dropped so much over the past two centuries compared to other hitherto much smaller countries such as England and Germany.