It is their biggest selling phone due to their logistical decision of how to ship
They have condensed the mass launch window to half the S3 window. Inherently this results in their biggest selling phone at launch as a result. The bloggers and media are parroting Samsung with minimum conjecture.
Agreed, the supply will be consumed, but seems their launch strategy is why the availability of larger models is also constrained. Production efficiency and expediency were obviously critical in shrinking the launch window so significantly. Impressive.