As a grain increases in size it's settling velocity increases.
The velocity of a settling grain in a fluid is controlled by a number of factors including the density of the particle, the density of the fluid, the fluid viscosity and the particles radius. This is derived from "Stokes' Law" - See the related link.
The terminal velocity (Vs) can be found using the following equation:
VS = (2 x r2(DP - DF)g) / (9 x VF)
Where:
r = Particle radius (m)
DP = Particle density (kgm-3)
DF = Fluid density (kgm-3)
g = Gravitational acceleration (ms-2)
VF = Fluid viscosity (Pa S)
From the above it should be clear that as the radius of the particle increases, the velocity at which it sinks will significantly increase (due to the square term).
Please see the related links for more information.