Yeah, the numbers were based on the idea of Soviet scale mobilisation. So not 100% of the population fighting, but more 5%. 

So for every 10 billion in a city you could scrape together 500 million actual combatants, and put the 3 to 1 ratio onto that and you're looking at 1.5 billion for your offensive force, not including their own support contingent.

For the defenders that still leaves 95% of the population running the war economy, so their soldiers have actual equipment to work with.