ecube: I am not a pixel peeper and my eyes is not as sharp as it was 40 or 50 years ago. But I do not understand why the comparison sample photos are of different magnification and resolution (pixel) setting, and lighting. I understand the difficulty in comparing two different SYSTEMS (i.e. Nikon v. Canon v. Sony), however, what is so difficult in comparing a D3100 v. D3200 using the exact PRIME lens (not similar) on the two bodies, using a jig to ensure exact distance and FOCUS, and identical resolution (pixel) setting, while balance, lighting and other camera settings. For the D3200, add shots at the higher PIXEL setting not available in the D3100. IMHO, the comparison methodology used here is acceptable for a high school science class.
The sample images are shown at 1:1 size (1 pixel captured = 1 pixel on screen), this is why they appear at different sizes. This shows the full potential of detail that the camera/sensor can pickup.
If they where shown all at the same size, this requires scaling down (extra processing), which usually means applying sharpening (for best results/to be fair) - and the std size would be something like 6MP for historical purposes - which would show no difference at all for modern sensors (pointless).
If YOU want to download the sample images and scale them to the same size (and apply appropriate sharpening) then you are welcome to.
"Lighting" as you put it, is affected by the metering, processing and colour rendition of each camera. DPreview uses standard lighting setup, distance from subject etc as best they can, and even provides the temp at time of testing (top of page).
Hope that helps take your understanding beyond HS science class level :).