I found this documented process to get the thin disk small again...but be aware very labor intensive and probably causes an I/O storm during the process.
Thursday, October 14, 2010
GETTING THIN DISK SMALL AGAIN
I was recently approached on can we shrink a Thin Disk after is gets expanded. Originally I didn't think this was possible. The scenario that was given was I have a thin disk, a large amount of data is copied which fills the disk and grows the this disk....then this data is deleted inside the guest vm.