Gravity does not care about size. Density mattters. With time, denser material will end up at the bottom. No matter the particule size. At the same density, smalle material will simply fill the "voids" that exist within the larger material.
Very true, but also take a look at nature. If you look at the delta and other water ecosystems, the finer particles do end up on top. They also compress to some degree. If you start with the coarser material and grade it upwards the finer material will stay on top. (Also consider how they build roads, big stuff on the bottom and grade upward).