That's merely a matter of better killing and transportation technology, and a much higher population to kill. If Mohammed had access to machines guns, artillery and tanks he would have killed far more unbelievers than he did.
No, it is the difference between having limits and not have limits. Islam recognizes limits; Hitler had none, save what he willed.
That's speculation. Facts, on the other hand, show that in the last century, with equal access to technology, the blood lust of atheist regimes have far outpaced those in which religion was permitted. This is both in terms of not only efficiency but philosophy as well.