Log in over SSH and run:

vmkfstools -c 50G -d thick /vmfs/volumes/san/vm/thick.vmdk
vmkfstools -c 50G -d thin /vmfs/volumes/san/vm/thin.vmdk

A ls will show:

50G thick-flat.vmdk
373 thick.vmdk
50G thin-flat.vmdk
398 thin.vmdk

However a du will show:

50G   thick-flat.vmdk
64K   thick.vmdk
1.0M  thin-flat.vmdk
64K   thin.vmdk

Which is, I think, rather neat…