lucky nation helps quite a bit but also they have tons of base tax as well as the NI that gives 20% manpower.
And elan and good wine.
But the province values are key as well as the fact that France can attack in any direction and the provinces are *worth* conquering.
Almost any country outside Europe has a less target rich environment of provinces such that they can grab up a lot of provinces that aren't all that great for tax, trade, manpower, production.
So France not only starts out strong, has vassals to annex that give them good numbers, but any direction they attack in is worth going for and helps make them stronger.
But the biggest thing of all is great wine - men don't win wars, wine does ...
(that last part wasn't true

).