The disparity is in the motivation; people are so quick to forget the human factor of war yet it is the most important factor as for most of human history it was the only factor.
On one side you have a force that is largely being forced to fight. Despite their advantages they largely and simply do not have the will nor the want to be where they are and to do what they are doing.
On the other side you have a force that is fighting for their lives, for their families, their freedoms, and their national identity. They will fight to the fucking last and if they run out of ammo they'll start throwing rocks.
Of course Russia was in for a bloody nose. It was stupid for anyone to expect anything less.
