Hitler and Stalin killed too many Russian people. Else a combined Russian - Ukrainian - Belorussian - Moldvin state would have had a population of 600 million today, and would have had twice as much power as the US is currently having.
It doesn't matter how many people they would have had, the communist system would have collapsed regardless. You only need to look at China with it's more than a billion people and it is not more powerful than the US to know that population, while important, is not the determining factor.