No conflict, neither in size nor in scope, is comparable to the Eastern Front, 1941-45. It was the deadliest theatre of war in history causing the death of over 30 million people. This colossal conflict played a decisive role in destroying the Nazi regime: the German armed forces suffered 80% of its military deaths in the Eastern Front. In the six months following Operation Barbarossa, 66% of the mobilized Red Army died or were taken prisoner and 90% of its mechanized strength was lost. Despite fears that Russia would fall, they prevailed, due not only to superior manpower, but a massive economic revival coupled with a reformation of the army and a powerful allies in Britain and the USA.
Manpower.
The German Army in the east drowned in "vast reservoirs of Soviet blood." Soviet tactics were appallingly costly in lives, of 34.5 million mobilized Russians, 84% were killed, wounded or captured. No other nation could have the attrition rate afforded to Russia by its vast population. The size of their manpower advantage was colossal, the Soviet Union's outstanding ability to replenish its losses meant that by October 1943 their numerical superiority ran at 2.2:1. A reserve service was obligatory for all under 50. On the eve of the German invasion, the Soviet Union had 14 million men with basic military training. This depth was not apparent to pre-invasion German reconnaissance and was key to their consistent regeneration of units in the early stages of the war. Fatal and persistent German underestimation of their enemy's strength played a crucial role in their eventual downfall.
Whilst Soviet manpower supremacy was not in itself determinative, their vast quantitative advantage did facilitate Russian shock tactics, allowing them to overwhelm isolated German divisions. Their large numbers afforded them a continuous front off attack, denying the Germans time to regroup. Despite suffering 1.2 million casualties at Stalingrad it would be unfair to say the Russian war effort was merely a "meat grinder.