I keep reading this on forums but I've yet to hear or read a technical explanation of this which satisfies me from an engineering pov.
FF normally performs better than smaller systems with a few exceptions... but when you look at images taken with FF cameras in crop mode do they suddenly drop a stop of dynamic range and develop more noise? I doubt it.
If the technology (FF v APS-C/other) is equal the only differences we should see should be due to different levels of image magnification. Possibly the only way to guarantee that the tec is the same is to shoot with a FF camera in both FF and crop modes and view the results and if you don't do that you'll almost certainly be comparing cameras with more differences than just the size of the sensor, the pixel size, circuitry and algorithms used may be different and indeed the imaging chips might not even be designed or made by the same companies.
IMVHO FF chips are generally better because of two reasons. They're generally the state of the art for the company producing them because they're flagship products and the manufacturers will have different design briefs and aims for different formats.
Doing those kinds of comparisons is difficult, because one stop of dynamic range and noise reduction is not huge in the great scheme of things, and then different cameras make drawing absolute conclusions tricky. DPreview takes a decent stab at it in the article, and comes to the same conclusion.
Two good comparisons are the Nikon D300 vs D700, and Canon 7D vs 5D2 - same generations of tech, and also with similar pixel counts between the two formats. However, the lenses would perform differently, even if they were the same lenses at different ends of the zoom range (to get the same framing). The lens problem would also apply to cropping down a FF image, and the pixel counts would obviously not be the same either.
However, ask any photographer that has moved from a D300 to a D700, or Canon 7D to 5D2 (and there are many of them) which is better, and you'll only get one answer. But, and this is where DPreview stopped short, the main thing most users want from FF is better sharpness. And that's what they get, for the reasons explained previously.
What is the technical explanation that you're missing? Light/photons is the raw material of photography - if you have twice as much of that to start with, then the camera can do a lot more. That's all there is to it in terms of ISO and noise.
