Small particles are subject to Brownian motion caused by bombardment by solvent molecules.
Larger particles have so much mass that no Brownian motion is observed.
Coagulation requires encounters between particles. Small particles may approach each other by diffusion, and Brownian motion may help overcome electrostatic repulsion so that they join together. However, as the aggregate grows, the mass increases so that Brownian motion becomes unimportant. Simple mixing causes the fluid to turn or rotate, and particles with their greater inertia may be subjected to centrifugal forces. It turns out that these of roughly of the same magnitude as Brownian motion.