What happens when the powder is free to move around more ?
You are making an assumption based on the premise that Grain 1 is different than Grain 2. They will have differing sizes, shapes and weights, but they are of the same makeup - which means they have the same hardness.
Ever see what happens when you rub two materials together that are exactly the same Rockwell Hardness? Nothing.
Umpteen millions of rounds are tumbled after loading to remove lube and give the round a finished look - even from factory ammo. You would have to tumble/vibrate the rounds for such and obscene amount of time to have detrimental impact on the powder that this discussion is kind of stupid really.