Wet or dry, the volume is similar. When you cut open the Sunshine Advanced Mix #4, the bag doesn't literally "explode" to double its size - - it stays at a constant volume.
If you threw hands-full of the stuff up in the air, then yes it might "expand" - but I personally think this is a misleading claim on their part. Just be aware to not buy one bag thinking you can have it fill a 6 cubic foot container.
Raybo