>>716191307A "Cluster" in this case is the size of each "Packet" stored on the chip.
Think of data being stored in tiny cardboard boxes. "Cluster Size" is how big the box is. You might call up more than one cluster to play a game, but you're always just pulling boxes out of that storage device and loading them into the RAM.
Having bigger boxes can sometimes fuck with emulation.
Having bigger boxes with ALWAYS fuck with games running on actual hardware.
For the 3DS, there's a real GBA in there. Having bigger cluster size than what's expected on a GBA cart means the game can't run.