GTX 590 reviews

[Sherman Tank]

you can't compare volts like that. you're comparing apples to oranges. the "why" is an electrical engineering degree or at least high school physics, but suffice it to say that even though the 590 uses less volts than 6990, it still uses more power.

It isn't really apples and oranges, they both use the same process, so both have pretty similar silicon, especially since Nvidia fixed their process issues.
The main reason Nvidia's chip uses more power is because they are allot bigger, and when your only running them at slow clocks (590) you basically kill performance/mm2, yet still have much more inefficient baggage like leakage from the extra die area dragging your performance/watt ratio down.

The thermal benefit of larger dies is that because the die has a larger surface area, it's easier to transfer the heat from the chip to the heatsink, thus combating issues with temps, due to higher TDP's.
Also if your not limited by TDP and you have a good architecture, then you can get more performance with a large chip Vs a smaller rivals chip.

The disadvantage of using larger dies, is they will generally be more prone to failure, similar to how yields are affected by larger dies, simply because the is more points of failure, and engineering the chip becomes harder, as the challenges of chip manufacture become amplified.

Personally I think the 580 cores are wasted on the 590, a couple 570 chips would have worked just as well, especially if Nvidia didn't fuse off the memory bus, so that it could use the full 1.5gb of Vram, or instead just doubled up the ram, they could have probably saved some cash using cheaper cores, and then spending the rest of the cards budget elsewhere.