Most common fillers and reinforcements have a micrometric size and must be used at levels in the order of 10% and more.From a strictly geometrical point of view, the size reduction of a particle, sphere or platelet with a constant shape factor, leads to an impressive growth of the ratio of surface area to volume (or weight) favouring the linkage and reaction with the surroundings. The following figure 'Surface/Volume-Ratio' is a log-log graph of the surface area to volume ratio of a particle versus its size, from nano up to micro scale. When the size is reduced from one micron down to one nanometer, the surface area is multiplied by 1000 for a same weight or volume with the affordable physical and chemical consequences.
Really, the effects are not so sharp as the following theoretical considerations show it. If we consider the reinforcement as the increase of modulus it can be modelled by several equations, each including one or several numerical or literal factor(s) concerning the particle size.