I see everyone arguing about the validity of the 20 million number, but what about the underlying question? Why were there so many more people, proportionally, in France than the rest of Europe?
Where are you getting this number from?
In addition to wanting to know where the 20 million number came from: my go-to resource for historical population is Maddison.
http://www.ggdc.net/maddison/maddison-project/data.htm
For 1500, it gives 15 million for France and 6.8 million for Spain, both of which numbers sound familiar from Lynch's "Spain under the Habsburgs" book.
I'm not an expert on France in the medieval times specifically, but I am familiar with some aspects of technology development in the medieval age. If we are looking for perspective on how and why France (or any European country X) has a large population in the Middle Ages, we should consider the so-called Medieval Agicultural Revolution in Europe.
Several advancements were made, including:
Now, the first two combined to enable the heavy plow, made of metal, and mounted on oxen. This allowed effective tilling of wet, heavy, deep soil of northern parts of Europe, which are mostly made of clay. This is what allowed countries such as France and England grow a much faster pace than other countries at the time (or, the equivalent areas thereof).
As mentioned by /u/idjet/ , asking specifically about France and comparing it against all other European countries at that time, is a false dilemma. It has similar population density as England, and more area.
There are also many other factors that can be considered, such as the black death's uneven spread and climate variations. Which you can go into more detail in the references below.
References: