I would expect them to be kept separately. Snapshots in Melee were kept as individual files on the memory card.Stage builder, videos, screenshots and all of the other features are probably included in this "128 blocks of save data."
If this is true, then it's absolutely amazing.
Also, can someone please explain to me why a large save file points to Brawl using a dual layer disk? Melee only used 11 block, but Animal Crossing required 59. Therefore, Animal Crossing used a dual layer disk. Sorry, but it doesn't work that way. The size of a game's save file isn't directly proportional to the size of its rom/iso. Animal Crossing was small enough to originally be released on an N64 cart.
FYI the Wii doesn't handle dual layer disks too well. Modders have been able to make the Wii read them, but only by adjusting a variable resistor on the drive to increase the output of the laser. This will eventually lead to a premature death of the Wii's drive.