Impressions of contrast change as a dependency of print size must be some sort of general visual artifact. I experience the same thing even in the electronic reproductions of the APUG Galleries.
How often have you seen at a thumbnail that looked like it had wonderful tonality, only to click and see the image go flat at its full uploaded size? Or seen a thumbnail that looked like it had way too much contrast, only to click and see a beautifully appropriate full range of tones from white to black?
For some reason if your goal is a full palette of shades of gray, it seems your thumbnails must look too contrasty. And that seems true even with the inevitable compromises inherent in electronic reproduction technologies.