It's not about the lack of pixels but the space it affords them.
The pixels are what? 10% bigger than the 5D3's?
ISO stops = 100,200,400,800,1600,3200,6400,12800,25600,51200 - 10 stops.
10% bigger pixels = extra stop of performance.
Hardly scientific and probably incorrect but we should really think about this before asking.
The flagship body will have the best of everything in, best circuitry, best capacitors, best chips, everything better.
There's a reason why the 5D3 is half the price.