In fairness it is both. AMD launched the 4770 on the 40nm process while the rest of the 4000 series was on 55nm to get a jump start for their next GPU revision.
Well you can do the same thing with the large chip too, just get less yields to begin with and fix the errors as you do with the smaller chips.
I don't think you will see that in generation, if we do then we are looking at a q4 or later launch for the big chips.