I am indeed also using a -R on the command-line.
coreftp.exe -R -s -u f:\bigtar.tgz -site fs1 -p /uploads/tar
I am using CoreFTP version 1.3 build 1417.02 and the file I am trying to transfer is approximately 10GB.
I have seen similar problems manifest themselves on other clients when the location that I am trying to resume from is beyond 4GB. However in this instance it didn't appear to matter where in the file I resumed from the % status value in silent mode always started at 0% and the resultant file was oversized.