I believe the smaller ones, being more rare, gave the greater rewards.
The Large Pieces, which could be mined up by anyone, even if they had
no Mining, gave the more common rewards.
But for the purposes of this event, one is no better than the other.
By having a piece in your backpack and casting Armageddon, you do
significant damage to the Shadowlords, and that is what gets you
the Cloak of Corruption.