The Russians lost more youth ("the best specimens of young [Russian] manhood") than the French did (even as a percentage of the population in WW2). That didn't make the Russia a nation of cowards. I think it has more to do with the liberal welfare society and socialism.
But Stalin had a talent for filling the front lines with peoples like Central Asians, Georgians, and Ukrainians, rather than Great Russians. And Communism was hardly less destructive of national will and morale than socialism in France. It would be interesting to see what percentage of Great Russians perished in WWII as opposed to percentages of races/nationalities from other parts of the Soviet Union.