When you spin it on the smaller axis, the centrifugal force on the phone only spins the weight within that radius between the axis point and the center of mass, whereas the longer flip deals with the uneven weight within the entirety of the phone from that longer axis point. Since the smaller axis point is more compressed, the force of mass is more distributed without the force dealing with everything in the phone from that point of force where you tossed it. The force you placed on the phone is where the centrifugal force actually pushes at.