Let me share an example of why sizing makes a difference. Attached is the same image sized according to the proportions of the D800, D700, and a web post.
This is the full image simply resized for a 1000 pixel width.
This is the image as shot with a D800E viewed at 100%. Said another way, this is viewing a 10 inch slice of the image printed to a width of 6 feet based on 300 dpi.
This is the image resized for the D700 proportions and then viewed at 100%. This is equivalent to a 10 inch slice of the image printed at 3 1/2 feet wide based on 300 dpi.
As you can see, the D800 is much more zoomed when you look at it at 100% and you can see some noise and softness that is not visible at the viewing level of a D700 size image even at 100%. By resizing to a common proportion, you can get a better idea of how the image looks in the same print or web post.
Resizing can work either direction as long as it is to a common size. If you resize the image from D800 to D700 proportions, you are essentially taking three pixels from the D800 and interpolating them into a single pixel. Some of these pixels may have some noise, but by resizing the noise is offset by pixels without noise so noise is not as visible.