They couldn't supply the troops they had. Their tank tracks were worn out by the time they got to Russia. They ran out of fuel. They ran out of spare parts. They ran out of food. When it took longer than expected they had no cold weather gear.It goes a little deeper than that. And this should be greatly noted for current world situation.
The Greeks did in the German's. Italy failed to crush Greece quickly as expected. Germany then had to send in many troops designated for Russia to get Greece.
The unplanned sending of German military forces to Greece, that were designed to go to Russia, likely was the true reason Germany didn't crush Russia. Had the Greeks succumbed to the Italians, Germany could have had a much more robust main effort fighting in Russia. I am quite sure our adversary in Asia knows this lesson very well.
And as Paul Harvey would say" And now you know the rest of the story".
Its unlikely more troops would not have helped. It was poorly planned from the start. They expected less resistance. They felt the Russian's couldn't produce modern weapons yet the Russian Tanks were as good as the standard panzer. Strategic failure - overestimate your competence and underestimate your enemy. Sounds familiar.
The fun thing about armchair general is you can make up whatever story you want.