http://prntscr.com/ftln7xGerman GDP was higher than Soviet GDP for most of the war. In 1945 at the very end of the war their GDPs were comparable. Even excluding Germany's occupied territories. And keep in mind the Soviet Union occupied territories of the former Russian Empire that factor into the total.
Also factor in that the USSR had far more natural resources and people than the German Reich. It's safe to say the USSR was far less efficient during the war and looking at Communist countries on a case-by-case basis, it does not appear that Communist countries are particular effective in war. Correlation does not imply causation sure but the claim "Communist countries are effective in war" is toothless.