This is how Xcode works when you download it from Apple's site, its in fragments. The way its mounted is the first fragment is uncompressed (from a BIN file) and is mounted (all of the fragments are also in the same directory). All the rest will be mounted along with it if all goes right.
Note: this is how it works with Xcode which is my only experience with fragments, I don't know if this will work for your situation.