> no obvious reason to have just half the population density of Germany.

France was historically always focused on Paris, because that was where the Emperor was. If you were not a farmer, there was little reason to live anywhere but Paris or other large cities.

In contrast, Germany historically consisted of thousands of small fiefdoms that each held some sort of local importance and each held authority of some sort. The Kaiser was pretty far away and only mattered in practice when the Kaiserreich was involved in some sort of conflict.