Why more resources into Army Group Center?

If Hitler was so concerned about the resources of Ukraine and the Caucasus (oil, wheat, minerals etc.) why was Army Group Center the largest of the the 3 army groups with the most men, tanks, resources, aircraft, logistics trucks etc.
It seems to me that Hitler should have put more resources into Army Group South rather than Army Group Group Center or at least make them relatively equal in terms of resources if he thought the south was more important than Moscow.