For one, the Allies had an EXTREMELY massive amount of production, which the German's desperately lacked around the end of Operation Barbarossa in the East.
Also, relating to production, the German's air force (The Luftwaffe) and Naval forces were essentially non-existent by the time the Americans landed in Sicily, but the Allies had a surplus of air assets (specifically bombers and Airborne infantry) which solidified an Allied victory in Europe.