Because the 1DMk3 has a larger sensor than the 7D (1.58 times greater area) and because it has fewer pixels (1.78 times fewer) then the 7D, the individual pixels on the 1DMk3 can be made significantly larger.
Although manufacturing innovations allowed the difference to be somewhat mitigated, the 1DMk3's pixels are reportedly still 1.67 times larger than 7D's (they would have been more than twice as large without these manufacturing changes).
It is my understanding that this specification accounts for much of the difference in some of the images produced by the two bodies.
Maxdave
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