The science fiction author Robert Heinlein was a socialist in his youth, but became a strident anticommunist during the Cold War. In 1960, he and his wife Virginia went as tourists to the USSR, and he wrote a magazine article about the experience, titled "Inside Intourist." He depicts the trip as interesting, but filthy, overpriced, and highly restricted by the presence of official minders. Virginia spoke fluent Russian, Robert just a little. They made a game out of evading control and seeing things they weren't supposed to see (like slums), and they made attempts to gather reality-check information on things like how much money people made and what their standard of living was.
The article was anthologized twenty years later, with an afterword in which Heinlein discusses at some length his theory that official figures on the population of Moscow were wildly overstated.
At that time the USSR claimed a population of 225,000,000 and claimed a population for Moskva of 5,000,000+... For many days we prowled Moskva--by car, by taxi when we did not want Intourist with us, by subway, by bus, and on foot. In the meantime, Mrs. Heinlein, in her fluent Russian, got acquainted with many people... she is a warm person. But, buried in the chitchat, she always learned these things: How old are you? Are you married? How many children do you have?
Meanwhile, Robert was looking at traffic on the river and comparing with shipping on the Rhine or the Panama Canal. He told Virginia that he thought the city felt to him like it had a population of 600,000 to 800,000, not five million. Virginia agreed:
"It's a lie. Unless they are breeding like flies everywhere outside Moscow, they have lost population since the War--not gained. I haven't found even one family with more than three children. The average is less than two. And they marry late. Robert, they aren't even replacing themselves."
Robert then says:
About a year later I had a chance to discuss it with an old shipmate, an admiral now retired...I asked him how many people there were in Moscow... "Make a guess. You must have some idea." ... He closed his eyes and kept quiet for several minutes. "Seven hundred and fifty thousand, not over that... I simply worked it as a logistics problem, War College style. But I had to stop and visualize the map first. Roads, rivers, railroads, size of marshalling yards, and so forth... The city just doesn't have the transportation facilities to be any bigger than that."
Is it possible that the USSR's official statements about the population of Moscow in that era were really inflated by an order of magnitude? It seems like modern demographers ought easily to have been able to notice such a huge discrepancy. Even a very crude post-Soviet census should have been able to detect it, e.g., by simply checking the number of old people in 1995 and seeing whether there were enough of them to account for the cohorts 35 years younger in 1960.