I'm sure this is an FAQ, and while r/askhistorians does not approve of the short answer, some stats. In June 1944 the Wehrmacht had 150 Divisions on the Eastern Front, and 66 on the Western Front. In Italy there were 27. In June 1942, the figures were 180, and 27, respectively, with three in Africa.

While the Red Army suffered massive casualties in the first 18 months of the war (around 60% of the total by December 1942), after this the casualty rates compared with the Germans were far closer. Analysis of the great battles such as Kursk and Operation Bagration bear this out.

While Lend_Lease and other supplies were helpful and filled important niches, or provided weapons at important times such as winter 1941-2 when Soviet industry was being relocated, in terms of weapons Soviet production was far ahead of Germany's and the most-produced combat aircraft and tank of all time are Soviet.

Aside from its military contribution, the greatest contribution by Western forces was in terms of massive economic drain imposed by bombing and the Battle of the Atlantic- necessitating submarine production- and associated blockade. Also, Western air forces broke the Luftwaffe in late 1943-mid 1944, reducing it to a nuisance force.