As I've said time and time again on these forums, the AI's inability to build any quantity of units is the main factor in this occuring. In my current game as Germany, while I terribly neglected building units and concentrated on research alone (Panthers by 1938), by 1940 I had 22 blitz divisions (panzers, mechs, mots) and over 50 infantry divisions. In the meantime, the AI had a total of 17 divisions in France (all infantry but 1 cavalry and 2 mots), and about 30 in the USSR, 1/3 of which was in the East facing Japan.