That is indeed a crushing Soviet victory; they're about 2 years ahead of schedule. Can't really imagine the circumstances that could lead to that! The Germans must have a High Tide mark of... -3? Normal German High Tide is over 20.
It's quite possible I misplayed something but I don't think so. At one point the Germans had over 20 points but when the High Tide marker came out they only had 11!
It all started in August 1942 when 1st and 2nd Panzer Armies surrendered in Voronezh. Hectic fighting ensued and then the Soviets unleashed their strategic surprise from the wasteland of Stalino. Over the next few months they managed to cut off and force the surrender of 4 more German armies. They continued to reward success by throwing everything they could upgrade into the breach - eventually re-capturing all their southern cities. They are on the doorstep of Greater Germany with just a German cadre and a single Rumanian army standing in their way. So I suppose it makes sense they capitulated.
I'm pretty sure its bad to let your best units surrender in this game