The USSR population figures consist only of Russian Soviet Federative Socialist Republic (RSFSR). In 1991 population of USSR was 293 M. According to this chart https://en.wikipedia.org/wiki/Soviet_Union#/media/File:Population_of_former_USSR.PNG the population of USSR was bigger than that USA's all those years.
in 1970, Pakistan had 123 million people, so it should be number 5. Also, I think you have mistakenly used the populations for the Russian part of the USSR rather than the entire USSR.