Oracle is designed for high performance, and Oracle does not clean up as rows are added and deleted, thus leaving a honeycomb of free space within the physical data blocks.
Especially with DLP, most of the space utilization is as BLOB (Binary Large Object).
In other words, after deleting incidents - you would start seeing free space in DLP UI.. System > Database > Table Summary. However the actual database file size would still remain unshrinked.
The only way, I've seen the file shrink especially for BLOB data is by performing a complete DB Export Import exercise. General information on this task overall:
http://www.dba-oracle.com/art_tuning_objects.htm
Hope this helps!