In my opinion the AI have trubble controling bigger armies. When I set up Armies of more then 10 div, Ai makes the front to large without considering other armies in the flanks. And spends half the armie on constant stratigic redeployment back and forward without considering to keep the corps diffrent div within hq distance.
Does that work better? Most of the time I use 5 Corps armies with 5 divisions each. I must always stop and redeploy my armies every 6 months to untangle the mess my AI generals have created. Their most recent stunt was redeploying the Manchurian army to India even though that was a different theater.