If you created a virtual disk in the thin format, you can convert it to thick.

The thin provisioned disk starts small and at first, uses just as much storage space as it needs for its initial operations. You can determine whether your virtual disk is in the thin format and, if required, convert it to thick. After having been converted, the virtual disk grows to its full capacity and occupies the entire datastore space provisioned to it during the disk’s creation.

For more information on thin provisioning and disk formats, see ESX Configuration Guide or ESXi Configuration Guide.