You'll have to wait for the hardware tear-downs to get a good sense about this. We know that both the Air and the Mini will have the A7 chip, but that doesn't mean that they're operating at the same clock rate (higher clock rate uses more power). The full-sized iPad has always had a larger-capacity battery than the Mini, but now that the iPad Air has been slimmed down it isn't clear how the battery capacity has been affected.