redbone wrote:It would make sense that larger grinds offer more resistance requiring greater tamp force in order to achieve the same result.
I think there's more to it than that, Robert. Particle size and shape are probably completely independent variables. The shape has to affect how tightly and how easily they'll fall into place, and the surface "finish" affects how easily they'll pack. These factors are probably more important than simple size in determining how closely together they can be packed and how much force it will take to achieve maximum compression. We know that some grinders produce "better" grinds than others. I suspect this has at least as much to do with absolute shape plus uniformity of shape and other characteristics of the ground coffee as it does with simple particle size (however you choose to measure it, since they're not perfect spheres so diameter is probably a poor measure).
If grounds were all the same size, perfectly round, and uniformly adhesive & cohesive, their size would be a major determinant of the force necessary to tamp to full compression. But they're not perfectly round and they're not perfectly uniform in shape. If some particles of the same maximum diameter (or weight or whatever you use to compare "size") have more - and, thus, smaller - facets than others, what we might call microdistribution will be drastically different from that of particles all of which have similar shapes and surface irregularities. And if the particle surfaces in those facets are rough (microscopically), they'll be harder to push against each other than if the facets are perfectly smooth.
If there's a high proportion of smaller particles (fines or larger - it doesn't matter, as long as they'll fit easily in the interstices among the big ones), the process of packing a tight puck will take less force even if many / most of the particles are large. There are now optical devices available to compare particle shape, and I suspect that they'll reveal a lot about the nature of grinding and tamping that we don't know.