looks like its expected. VDP takes back up of the total VMDK first time not the data in it.
From VDP perspective the VMDK is grown to 11Gb and not decreased the size after deleting the data. Thin disk are not intelligent enough to shrink automatically when the data deleted.
You can go for SE Sparse disk to achieve this. SE sparse disk will shrink itself when the data are deleted. Its implemented in 5.1, i have never tried it. You can find more details in following articles.
http://www.virtuallyghetto.com/2012/09/how-to-create-se-sparse-space-efficient.html
http://cormachogan.com/2012/09/05/vsphere-5-1-storage-enhancements-part-2-se-sparse-disks/
