We had a chance to compare two cards, at least in measurements and we found out that 12.4 card PCB is almost the same like the 9.5 one. Both cards have circa 9.5 inch PCB with some different components.
This means that the cooler makes the size. The OEM card, also known as 12.4 inch one is as big because it has bunch of copper and heat pipes on a big cooler while the 9.5 inch card has much smaller cooler.
I am sure that 12.4 massive cooler can remove some massive temperature out of this horribly hot card and that is exactly what OEM's wants.