I am no expert on emulsions, but I think that film grain can be divided into the old type which is more or less cubic in shape and the modern "T" type that has more of a flat "pancake" form. The "cubes" vary in size with different manufacturers and film speed. More agressive developers partially dissolve the edges while other film/developer combinations tend to act more gently and maintain the sharp "corners". I am sure there will be a more scientific explanation.