Lancaster's Square Law
Mind, however truth that may bring that does not address the
Concentration of power the Germans were able to bring to bear, as well as
Volume of Fire their artillery and infantry had that the Russians lacked.
The Germans have better led, more concentrated armored spearheads. Their infantry units generally have more available squad-based machine guns and were generally better trained (this is not to say Russians only did human wave tactics). Tactically, the Germans have a higher volume of fire and is more likely to surpress and eliminate opposing members both due to the larger numbers as well as lower quality of both weapons and training. Strategetically, the Germans are attacking and thus were able to determine the focus of campaign, leaving defending Russians often off balance and unprepared, cutting through lines and surrounding pockets of resistance to be dive-bombed into submission.
The underestimation is less about numbers of people. The thicker the hay the easier the mowing from defensive positions and behind tank armor, watching for the right moment to slash through the front lines and to cut off rail lines and logistics.
The underestimation was more about how quickly the military was able to recuperate from Stalin's purges, about how rapidly factories moved to the trans-ural region were able to set up and produce capable and even advanced tank designs. The underestimation was how tenatious the defenders were, how little anti-communist resistance they were able to find, and how successful the USSR was able to unite against the German Foe instead of collapsing like a house of cards.
The biggest argument IMHO is whether the Germans had EVER had a chance at the start. I truly believe that they had a chance, but the plausibility?
To win...
The Germans must have captured the key symbolic cities of Leningrad, Moscow and Stalingrad.
The Germans must raise significant amounts of anti-communist or proto-nationalist resistance against USSR.
They must kill, capture, incapitate or depower a significantly large amount of skilled officials to decapitate the government.
They must prevent the activation of Patriotism and promote depression, passivism, or outright cultivation of Quislings.
They must force an semi-official capitulation, possibly by a Soviet Exile Government or a counterrevolutionary Russian force.