It seems in most novels that the Imperial guard and countless innocents are slaughtered by orcs, chaos, tyrranids and the like, yet the total population of humanity is never depleted.

This questions asks if humans are altered in some way, hinting that it might be why the Imperium of man is still going strong. Is this the case, or is some other factor at hand? Is this addressed in any of the books/games/other media?

How did the Imperium of Man become so well populated?