Were these datastores created with with an earlier version of vSphere? With vSphere 5 block size is no longer set and there is no soze limitation other than 2 TB -51s B for a vmdk.
I am thinking you are have your VM stored on a VMFS 5 datastore because you would not have been able to expand the virtual disk with VFMS 3 datastore - How much free space is on the datastore?
Storage vMotion would be the best way to move the VM or you could cold migrate -