If what you explain is really the case, thenI'd consider this a huge bug, and you should open a support case with VMware.
The maximum growth of a thin provisioned virtual disk when deleting a snapshot should not exceed the snapshot's size.
How did you verify the virtual disk sizes?
The most reliable way to do this is to run ls -lisa from the command line. This command will show the provisioned size in bytes, as well as the used disk space in kB (second column).
To verify, I browsed the files on the datastore via the vSphere Web console. I also SSH'd into the host, navigated to the vm on the datastore, and ran:
ls -lh *.vmdk
and then to verify actual usage,
du -h *.vmdk