How was that supposed to work?

What I always wondered was, you take a Conquering Nation, pretty much any from history, and what they're doing is conquering the world, yeah? But the world is quite large. And Germany is quite small. So how can there be enough people in Germany to conquer the world? Because the troops are streaming out of the home land, right? And, unlike Risk, I'm pretty sure you're not going to start generating troops from the conquered nations. Historically, conscription may have been a thing, and it becomes in the best interest of those conscripted (re: enslaved) to crew the ship, thus preserving their own lives, but that in more modern times, putting guns in the hands of your freshly subjugated enemies and hoping they'll get your back in the next battle might be one of the very last things you (would want to) do. So what's the theory? Kill every man who could fight you, hope that the next generation thinks of your new empire as their home, as opposed to their father-killin' enemy?