change disk space settings within vmware

Hi,

I have done a fair bit of research but haven't been able to find a
concise answer.

The VM guest OS is RHEL and has disk space of 40GB (checked this via the OS using df -h).

Within vSphere when viewing the settings the disk is 'Thick' provisioned.  Total provisioned space is 600GB and used space is 200GB.

I plan to move this VM to a new DC using the VM Convertor.

I'd like to reduce the total provisioned space to 120GB.  What's the best way to do this?

Why is the used space at 200GB when the OS is using 40GB?

Many thanks.