A sparsebundle like many other counter parts in virtual computer environments, does not shrink its size even if you free-up the available space. You need to use hidutil to do it.
hdiutil compact /Volumes/Backup/mjhsieh.sparsebundle
August 07, 2011
Shrinking a sparsebundle
Posted by mjhsieh at August 07, 2011 06:57 AM