We were fighting Japan and Italy too. The Russians had more fighting early because they were in it earlier. So were the British.
According to Cornelius Ryan who wrote several accounts of the war, Hitler devoted most of his resources to the West after D-day. He always underestimated Russian strength.
I would also suggest that our air campaign along with the British did as much as anything to winning the war. We also suffered very heavy casualties in the air war.
With Russia as an neutral entity instead of enemy, he could have held most of Europe, and I believe would have eventually been able to negotiate a surrender with Allied Forces, retaining control of his new nation. Instead, his ego and his need for complete control crippled his military campaign, and thus contributed to the Allied victory.