vmkfstools -i will clone the disk to the exact size . but again we could change the descriptor value and then clone which would result the disk size to be shrinked. but its not advisable and could result in corruption.
I would recommend to use the VMware converter ( free software from VMware ) which is the best option. And the best part you can do it on the fly. where as vmkfstools would require a downtime.
As Avinash said use the converter. I did it using vmkfstools once for doing this and ran into multiple issues which included corruption. But i was working on a test machine. To be safe use the converter.
Or even goto the VM folder, select and right click the .vmdk file.
You will get and option "INFLATE". Inflating will make the disk from thin to thick. But the VM needs to be powered off