D800 has problem with camera-shake, 5d3 does not. Why?

Several reputable Nikon reviewers of the D800 have commented on the difficulty of hand-holding the D800 without getting movement blur in their images (e.g., http://mansurovs.com/nikon-d800-review ). They recommend using higher shutter speeds than one normally would.

I have not encountered this difficulty with the 5d3, nor have I seen it remarked on by anyone elsewhere.

Why is this? Is the problem with the D800 and camera-shake a by-product of the smaller, more densely packed pixels on the D800 sensor compared to the 5d3?

Or is there some other factor with the D800 that is producing a tendency to shaky images when hand-held with normal shutter speeds?

Also, if it is the pixel density that is creating the tendency to hand-held movement blur, would the same factor also increase the extent to which any movement by the subject results in a blurred image?