A virtual disk in thin format uses only as much space on the datastore as needed. This means that, if you create a 10 GB virtual disk and place 3 GB of data in it, only the 3 GB of space on the datastore will be used, but the performance will not be as good as with the other two disk types.
You can convert a thin disk to a thick disk by inflating it to its full size. Here is how to do that using vSphere Web Client:
1. Shut down the guest OS.
2. Browse to the datastore on which the virtual disk is stored and find the disk’s .vmdk file. Right-click the virtual disk file and select the Inflate option: