Why are the D1-4 so big?

Aaron WillisChang wrote:

So I had pulled out the D4 in front of some non-photog friends and of course, as usual, were cries of "wow that's a big camera" (even though I had a tiny prime on it).

But then someone asked, "why is that so big?" and I answered, "they just make them that way," and she persisted, "why do they make them that way?"

And I had no answer.