Russia did pay a massive cost for WW2, but they also paid a high cost due to: 1) their attempt to placate the Nazi’s with their non-aggression pact and attempt to avoid war (which they weren’t that prepared for), 2) the Great Purge in the late 30’s where they executed and imprisoned a massive number of experience officers, which they had to replace, 3) their tactics (no retreat) which resulted in massive encirclements and POWs who were starved to death.

Eventually the USSR learned and with support of the UK and US in terms of materiel turned into a formidable fighting force. But a decent amount of their casualties were somewhat self-imposed.