SUMMARY
When restoring a thin-disk VM that had a snapshot at time of backup, the restored VM disks are created as thick
ISSUE
When restoring a thin-disk VM that had a snapshot at time of backup, the restored VM disks are created as thick on vSphere 4.x. This is a known issue with VMware version 4.x:
When a virtual machine with thin disk has a snapshot, the redo log is incorrectly created as thick disk
The bug was fixed in vSphere 5. For version 4.x, you can first commit or remove the snapshot before taking the backup.