WWII was Russians fighting for survival with the support of the rest of the USSR at a time when their population was growing and Stalin had an iron grip on power.
Russia today is shrinking, even before the war, which is a relatively unpopular war of primal conquest instead of self-defense. Back then the Kremlin has international support, including a ton of equipment from the US, and the Soviet dream of a better world. Today's Kremlin is isolated, quickly burning through USSR leftovers, and scared to start another mobilization because the last was deeply unpopular.