Despite the fact that Intel's desktop Sandy Bridge-E lineup tops out at six cores, most of the chips are based on eight-core silicon. Only needing six working cores out of eight surely improves yields, and disabling the extra cores allows Intel to hit higher clock speeds while staying within the family's 130W thermal envelope. Eight cores require a lot of die area, though. SB-E silicon measures 435 mm², more than double the size of a quad-core Sandy Bridge chip. That translates to a lot of wasted die area for Sandy Bridge-E's quad-core variant, the Core i7-3820—or does it?
VR-Zone is reporting that the i7-3820 uses a different die than other SB-E CPUs. The native quad-core design is said to be about two thirds the size of the eight-core die. It's not closer to half the size because Sandy Bridge-E's quad-channel memory controller, 40-lane PCIe 3.0 component, and other logic blocks haven't been touched. SB-E is a modular design that allows Intel to easily trim slices of the chip containing cores, cache, and stops on the ring bus.
Using a native quad-core die should make the Core i7-3820 and its inevitable Xeon counterpart cheaper to produce than if the chips had been based on the older eight-core silicon. I suspect the quad-core die is more power-efficient, as well, but the i7-3820 has the same 130W TDP as the rest of the line.