What has always amazed me about Stalingrad was the enormous armies the Russians used in their riposte.  All of the really bitter fighting was done by Chuikov's 62nd army before the encirclement, and he fought them nearly to a standstill.  And yet on top of this brave force was a much larger encircling force held in reserve. It was so large that it easily fought off all attempts at relief.  It seems that, by the time the encirclement began, the Russians were already much stronger than the Germans, and that battle for Stalingrad was just the symptom that let us all see that the Russians were able at that point to build up their strength at a much greater rate than the Germans.

 Is this point of view correct?  If so, the mystery to me is *how* in the world did the Russians manage to not only

keep it together, but even outgrow the strength of the Germans with half of their country, the most productive half, either gone or under siege?