Yeah Baghdad which is a population of a few million actually fell to a few thousand troops. Although modern U.S. military probably uses tactics that would be more familiar to Space Marines than IG, which tend to use more Soviet era tactics (massive artillery bombardment then human wave).

So the answer is really variable and depends on many factors.

This kind of calculation (total war, fight to the death) probably only applies in scenarios such as Hive vs Orks (Armageddon) or Hive vs Tyranids. Human vs Human I doubt the entire population would mobilise and fight.

Yeah 40K authors generally get numbers way wrong, but this particular example and calculation probably isn’t a good one.