Thirty odd years ago, when I was doing a degree in photography, we were told to photograph the same still life scene on 35mm, 6x6, 6x9, 4x5, and 5x7. We then printed each of these to 5x7, 10x8, 11x14, 16x20 and 20x30. The bottom line was that even with small print sizes you could still see quality advantages from using larger film formats.
Sure, lower quality optics and deteriorating film flatness removed some of the benefits of larger film sizes. And for prints of 10x8 or smaller 35mm was good enough for most applications, and 6x6 for 11x14...and so on. But there was no denying that if you were chasing the absolute best in image quality, larger formats were the way to go, rght up to contact prints which delivered the absolute gold standard.
However, with digital, in particular when the final output is an inkjet print, I just don't see the equivilant of a contact print. In other words the quality advantages of more pixels doesn't slowly fall away like the quality advantages of bigger film formats. Digital quality just seems to hit a brick wall and stop dead.
Here's an example. If I photograph the same scene using a Canon 5D and also using a Phase One P25 back on a Linhof with a sliding carriage (giving 40+MP for the stitched shot), and then print them both out to A4, I can see absolutely no difference in quality whatsoever. I've got to go up to A2 prints before I can see real differences (and I'm sceptical that even here these differences would be noticed by non-photographers).
Those of us who learnt our craft with film have taken on board an assumption that "bigger film means better prints". But with today's inkjet technology I'm sceptical that you get any better quality from small or medium sized prints (say A4 or A3) simply by throwing more pixels at the challenge. You just reach a point where there's simply no further benefit for a given paper size from more pixels.