Helge, correct me if I'm wrong, but I suspect your definition of 'tonality' is simply the way in which brightness in the subject is translated into density in the image? I think the rest of us are using the word in a broader sense to mean the aesthetics of tones in the finished print, as Matt describes. I was particularly thinking of the smooth transition from one tone to another around surfaces like e.g. an egg.
Your initial point was that the rendition of tones in the negative takes place on such a small scale that a change of negative size is irrelevant. So I imagine you would say that screen images, where smooth transition of tones has everything to do with pixel count - is a false comparison because a pixel is still large compared with the screen image, whereas a silver grain is tiny relative to either negative or print?
I've never done this, but I imagine that if I photograph an egg on 35mm FP4+, 6x6 FP4+ or 5x7 FP4+, the transition of tones around the surface of the egg will look smoother in both negative and print (more so) from the larger formats. You describe this as a fallacy, if I understand you correctly. Within the 35mm format, I have used films with different speeds (and therefore grain size), and a range of different developers and development times (and therefore grain size). That experience supports what I'd always understood, that the finer the grain size in the negative, and the lower the magnification in the print, the smoother the transition of tones appears. I expect this trend to continue as one steadily increases negative area, with the accompanying decrease in magnification for the same size print. Am I deluding myself?