smbclient's built-in tar feature lists the size modulo 8 GB. It effectively 
truncates the size from left if it is >=8GB. For example, a  file 
14,570,219,438 bytes big (octal 154434763656) gets listed as
 5,980,284,846 bytes big (octal 54434763656).

The good news is that it puts all 14+ GBs on the tape but the tar can only
recover those cca. 6 GB listed, and complain about "obsolescent base-64 
headers".

As of GNU-tar 1.13.25 (on my SuSE 8.2) the correct way to list size of files 
greater than 11 octal digits is to put binary long 0x80000000 in the first 4 
bytes of the field and a binary long long size in the remaining 8 bytes, 
trailing '\0' be damned. Although I didn't find any documentation to that
effect, this is how it works.