Source: Direct comparison of Moscow vs. Saint-Petersburg populations.
On the eve of World War I, Saint-Petersburg was bigger than Moscow: 2.1M to 1.8M. It had a more developed and sophisticated economy, and was drawing in more people, though this was partially canceled out by the higher fertility rate in more religious and traditionalist Moscow. SPB had an order of magnitude more financial activity, though Moscow was a major manufacturing center. Perhaps also bolder plans on infrastructure development; construction on the original Moscow Metro began in 1914, while SPB had no metro plans at the outbreak of WW1. There were some discussions in Nicholas II’s circles about moving the capital back to Moscow, in line with neo-Muscovite artistic sentiment. Long shot, but if that had happened, Moscow and SPB would have ended up truly level pegging. But otherwise, the ~80% ratio between them would have likely remained.
Relative population of Moscow vs. Saint-Petersburg.
After Bolsheviks moved capital to Moscow, SPB and Moscow swapped positions while keeping the same approximate ratio, though now loaded in favor of Moscow.
However, it was really the Siege of Leningrad that permanently displaced the “northern capital” as a competitor to Moscow. Perhaps that is its single biggest Big Historical impact: Raising Moscow’s relative lead over SPB from ~25%, to ~75%-100%.
Ever since, SPB has been been more Russia’s biggest millionik than its “northern capital”. The post-Soviet period beat in the last nail in its coffin; many more repatriates from the former USSR, as well as Russians moving in search of higher quality of life, went to Moscow, as opposed to banditized SPB. As a result, whereas Saint-Petersburg has increased by a factor of just slightly more than 2x since its pre-revolutionary population peak at 2.4M in 1916, Moscow has exploded sevenfold. Consequently, Moscow now completely overshadows SPB in economics, politics, and science production, with SPB only remaining competitive in culture and tourism.
In the context of the Sixth Proof of God theory, perhaps this (relative) collapse was divine punishment for having hosted the Bolshevik Revolution.
Incidentally, if you’re in Saint-Petersburg, one sight worth passing by is a monument to the cats Elisey and Vasilisa (Malaya Sadovaya St, St Petersburg, 191023). If you manage to throw a coin onto the ledge holding them, you will increase your character Luck stat. I was told that they were commemorating the countless Unknown Cats eaten during the famine imposed by the German blockade, though what I now read on the Internet is that they are actually tributes to Siberian cats brought into Leningrad in 1943 to control the city’s rampant rodent population, which had exploded in the absence of their natural predators in the previous two years.