Surface area is of utmost importance. The larger the particles in colloidal suspension, the less surface area is available to interact efficiently within a biological system. If you take a pure silver dollar, you basically have three sides and very limited surface area. If you release that ounce of pure silver into a trillion (or more) little active particles (Ag+), then you have surface area orders of magnitude greater than conceivable. When it comes to the active state of silver, smaller is better.
