Morale isn't as much of a problem as food. POWs in Germany were fed very poorly to begin with, and that amount only declined as the war went on and Germany's resources waned, along with the quality (for example, ersatz "bread" which was 50% actual bread and 50% cellulose and manufacturing byproducts). On the contrary, POWs in the USSR received more and more food as the war progressed. POWs in the USSR also received tobacco, soap, and vitamin-rich fish oil.
Sources (with lots of detailed tables)