system
Given the following post, respond as if you're commenting on Reddit:
human
[Marvel] What is happening "behind the scenes" of Sandman's power that he's limited specifically to Itty bitty pieces of stone? Why can he manipulate as much sand as he wants, but when it comes to gravel and rocks he just can't do anything?

Side question.  Could he crush rocks and gravel into sand, and then be able to control it, or is it still earmarked as "not sand"?
gpt
He has a very unique form of geokinesis, whereby the total force of his power dwarfs almost any other geokinetic, but the force he can exert on any one object is miniscule. He has very little strength, but lots of fine control, which allows him to manipulate millions of little bits of sand in complex ways, but stops him lifting anything much bigger than a few grains stuck together.