This is most likely related to the RDM pointer .vmdk file which still contains the old size information. This is basically not an issue for a physical (pass-trough) RDM - except for the incorrect report - , but you could run into issues at a later time when you e.g. try to convert the RDM into a virtual disk. The cloning process might just consider the number of blocks in the mapping .vmdk file rather than the real LUN size.
Unless I'm missing something, the only option is to recreate the mapping file, which however requires downtime.
André