Size distribution - Real solder powder is not monodisperse (single diameter), but has a spread of sizes (typically approximating to a log-normal distribution). Generally, the wider the distribution, the higher the maximum packing fraction. This can be easily understood by looking at the picture from the last post (below) and imagining tiny spheres fitting into the little interstices between the particles: they would have to be around 1/10 of the diameter of the larger spheres. Theoretically, you can get 0.99 or even higher packing fractions with a very specific multimodal distribution, but you never see this in real life.
Boundary layer - Every time a fluid flows over a surface, the part of that fluid closest to the solid surface does not move, relative to the surface. On a molecular level, individual molecules are diffusing in and out of this so-called "static boundary layer", but essentially, the fluid right next to the surface is completely immobile. The fluid just above this static layer is moving slowly, and the next layer out moves faster still, until the velocity is the same as that in the bulk fluid. "Ok" you say, "so what?" Well the fluid around the solder particle therefore forms a kind of shell that is almost like an extension of the particle into the fluid, and the thickness of the shell is not dependent on the particle size. Complicating things further is the fact that solder paste fluxes are plastic, not Newtonian, so the boundary layer goes out even further. To sum up: smaller solder particles have a "virtual shell" around them that means you need a lower metal weight percent to get the same rheology.
Non-sphericality - The sphere is an ideal solid, and any deviation from perfect roundness causes an increase in the "k" factor, which is 2.5 for spheres, and increases as the particle become sincreasingly deformed, more and more fluid being trapped either within or around the particle (see picture). Most powder these days is spherical, so it's not a big deal
Chemical reactions - Activators (see previous posts) are very good at removing oxides from metal surfaces. There is a myth that there is a magical temperature at which the activation (metal oxide plus activator) reaction occurs, but that's exactly what it is: the myth of the "activation temperature". How can I demosnstrate it's a myth? Simply because solder paste has to be stored in a refrigerator, or else it increases in viscosity with time through the so-called "concretion" reaction which is just the slow reaction of activators with metal oxides to form solid reaction products, just in the same way that water hydrates cement to swell the crystals and cause them to change shape and grow, interlocking together into a solid mass.
Air - No matter how hard you try, you will always have a little air mixed in with your solder paste.
A complicated answer to a simple question!
Oh, and by the way, once you've embarked on a study of solder paste rheology, there is is the little matter of "artefacts". A subject for another time....