Eric is explaining what DxO does to come up with a common print size for comparison purposes.
I downsize in order to view different camera photos on the computer at the same size for comparison purposes.
I use a D3X which has an image size of 6,048 X 4,032 pixels or 24.4 mp.
I compare the images to my D700 with is 4,256 X 2,832 pixels or 12.1 mp.
I do this by viewing my D3X photo in CS5 and looking at the image size of the photo. It shows 6,046 and 4,032. I change that to 4,256 and 2,832. And save it. If my D3X and D700 photos were of the exact same scene I now have two photos of the exact same size and I can compare the quality of both to each other. If I'm looking at the noise of the D3X photo at 100%, the noise is slightly reduced when down sizing to the D700 size.
Unlike the DxO results which show the D700 noise as slightly better then the D3X noise, I find the D3X has ever so slightly less noise then the D700 after downsizing on my computer screen. Here is a comparison of my D3X and D700 side by side after down sizing.
The D3X photo is on the left. This is a comparison of photos taken at ISO 6400 and enlarged 100%.
I get the same results every time I run these comparison tests.