I saw some here quoting the Soviet estimated losses on the war, excluding civilians of course, is 6.6 million. It is an under estimate by at least 2 million.
Also, the significant of the western fronts, beside the actual number of German casualties and troops occupied with that front, was logistically and operatively immense.
The Naval blockade on Germany forced created critical shortages in many types of resources and commodities and almost shut completely their international trade. It is true that thanks to the so called neutral countries Sweden and Switzerland The Germans could keep trading with the outside world but only to a very limited and crippling level. Moreover, the sea war took immense resources and casualties from Germany. On the other hand, the soviet could get supply for free, including the shipping.
The air campaign severely crippled the German industrial production and transportation. They managed to increase production despite that, but it was still a huge handicap. Without it they could produce and ship to the front allot more supplies and allot faster.
There were Immense logistical losses to the Germans in the African front. Most of the equipment the Germans sent was sunk on the way. And the battle of Britain also took a huge price for the Luftwaffe.
If the Germans had to fight a single front war with no blockade, my opinion is they would have subdue Russia by 1942 at the latest.