[othello]

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.

1. https://en.wikipedia.org/wiki/Demographics_of_France