You're comparing a volunteer army in peacetime with a conscripted army fighting a total war on two fronts. In 1944, the Nazis had been at war for 5 years and had spent a further 5 years before that militarizing German society.

Before Hitler broke the Versailles treaty in 1935, the German army was limited to only 100,000 members -- fewer than today. It took less than a decade to mobilize to the extent you describe. It happened then, it can happen today.