Yep, Liam is correct, airfleets are not the über-unit of the game: they are required, but in proportionate amounts.
If the Axis has played well, when Barbarossa begins, Allies have some edge on MPP-income (Allies perhaps 480+136+180=796, and Axis could have 550+140=690), but Axis have more accrued units and experience. The Axis amount of units is that great, that without a significant bid to help Allies/Russia, the Axis will win. Of course, if your play proceeds along these lines, you will need a bid to balance the game.
But over-concentrating on AFs will detorioarate your game. Once the fronts go from North to South in Russia, and similarly in France at the same time, both parties need to have ground units for that. Otherwise one can push through and make cheap attacks on the too many AFs, which in turn is very costly for the owner of the AFs.