Modern history gives significantly lower estimates for those numbers, it was likely less than 2 million people in France. At the same time the Mongol's army was probably about 20k people, not 500k as claimed.
Are you sure you aren't thinking of post plague Europe? I just double checked wikipedia and it basically says between 13-20 million people in France depending on what you consider to be "France" (modern borders are closer to 20 million).
