Well. this can be done by doing V2V conversion at the last summary wizard you can edit the disk size to less than current . Doesnt matter its Thin or Thick.
All the best,
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you have two choices if you want to just reclaim the space first zero out unused space with e.g. SDelete utility from within the guest OS and then use vmkfstools -K <path to .vmdk> command (from ESXi shell) to punch zeroes this will dealocate
all zeroed out blocks and leaves only those which contains valid data. --Before this operation shut down your VM--
Another option (after in-guest SDelete operation) instead of using vmkfstools is to use Storage vMotion and move VM to another VMFS datastore this will also reclaim null blocks. (for this method to work both VMFS datastores must have same block size)
If you want to permanently resize/reduce thin disk use Standalone Converter choose P2V/V2V conversion ...in destination location options under Data to copy...choose "Min size" in destination size column.
Storage vMotion to another another datastore, V2V involves down time, and this depends how big your VM is. If you are thin provisioning on the storage back-end then you should not really thin provision VM (thin-on-thin) as general rule.