How a transformer works and apply those rules to a transformer where the primary core cross-sectional area is twice.
The flux density (B) is a linear function of the current for both the primary and secondary. Since the primary cross-sectional area is twice that of the secondary, it produces twice the flux (BxA) at the same current. So to cancel out the secondary flux of a 1:1 transformer, the primary only needs to drive half the current of the secondary. This means VxI of the primary is half that of the secondary creating the impossible result of efficiency equals 200%.