Just to say it, because it is one feature MS Virtual Server and Hyper-V have that was missing ESX in GUI vCenter... the article :

I was always very happy with one of the features present in VMware Workstation since few years now. Actually you are able to specify in VMware Workstation when you configure new virtual disk how big the disk will be. But when you specify for example 50 Gigs virtual disk inside Workstation , the disks real space on your PC gets just few hundred Ko. The vmdk file which is laying on your hard disk will “auto-grow” as you install the system and the applications etc…

Sometimes I wish there was this possibility in an ESX environment, but unfortunately that is not possible, or is it? Yes.

The way it works is that you have to create a VMDK file for your virtual machine through a command line. (in vSphere 4 this can be done via VI client… )

This is because you cannot do this from the GUI of the Virtual Infrastructure client. For example I’ll show you how to create a VMDK file (your hard disk in ESX environement) that would grow until 20 GB but only take up the space that it really uses.

Here is how:

1.) Login to the Service Console of the server or remotely via putty (how to enable SSH and login remotely from putty)

2.) Type in  “vmkfstools -c 20G -d thin /vmfs/volumes/datastore/virtualmachine_name/virtualmachine_disk1.vmdk

3.) Then you can login to the VI client and add the existing disk you just created to the VM template.

http://www.vladan.fr/thin-provisionning-save-diskspace-with-esx/