Mac OS X supports a variety of disk image formats. When you create a sparse disk image, it has a maximum size and grows as you add files to it. This lets you conserve space. DMG files are compressed disk images and also conserve space. If you need to convert a sparse image to a DMG, you can use the Mac's Disk Utility program to convert the image.
Double-click the sparse image to mount it on Mac OS X. Open Disk Utility by clicking "Applications," "Utilities" and "Disk Utility."
Select the sparse disc image from the list of volumes on the left pane of the window. Click "Convert." Enter a name for the DMG image that you want to create. Select "Compressed" from the "Image Format" drop-down menu.
Click "Save" to convert the sparse image.