.SPARSEIMAGE File Extension
What is an SPARSEIMAGE file?
Disk image file type created with Apple's Disk Utility program (included with Mac OS X); similar to a standard Apple Disk Image (.DMG) file, but only takes up the amount of space of the data contained within the disk image.
To create a sparse image file, click the "New Image" button in Disk Utility. Then choose "sparse disk image" from the Image Format drop-down menu.
NOTE: Apple's FileVault feature initially used encrypted sparse image files to secure a user's home directory. Newer versions of FileVault use the .SPARSEBUNDLE format.