I think that France is rather united. Bretons, for instance, are proud of being Bretons, but feel perfectly French too, apart from a very anecdotal minority. The only part of France (outside oversea territories) with some people who are serious about independance is Corsica.
I didn’t mean separatism, just groups that are in France and French by citizenship (and often language) but not ethnically French. The Corsicans are another good example.
The heartland of Russia is a huge wide-open plain in the middle of a vast land mass, with few natural barriers to invasion from any side. It has been invaded time and time again in history (Mongols, Swedes, Poles, French, Imperial Germans, Nazi Germans), and some of those invasions–especially the Mongols and Nazis–have been pretty horrific.
It has been speculated that this fact of geography makes Russians more willing to accept ultra-authoritarian rule, for their own protection.
And let’s not deny the facts. One of the people who invaded Russia’s heartland is the Russians themselves. They originated in what’s now Ukraine and Belarus. Most of what’s now inhabited by Russians is territory that was conquered from other people.