When does UNIX “defrag”? (specifically Solaris)

Very interesting finding. Although I've never used ls -s to check whether a file is empty or not, I would have assumed, that it reports 0 for empty files, too.

To your question: As Mat already commented, show them your test results. To explain the results to them, state that ls -s reports the amount of allocated blocks in the filesystem, not the actual size in bytes. Obviously some filesystem implementations allocate blocks even if they don't have to store any data instead of storing just a NULL pointer in the inode.

The explanation for this may be performance-related. To create empty files that will stay empty is an exeption for normal processing (the most common usage I've seen would be the creation of status files where the existance of a file represents a certain state of the software).

But normally a file created will get some data soon, so the designers of a certain FS may have assumed that it pays off to immediately allocate a data block upon file creation, so when the first data arrives this task is already done.

Second reason could be that a file has contained data in the past which has been erased. Instead of freeing the last data block it may be worthy to keep that data block for reuse by the same file.

EDIT:

One more reason came to mind: The filesystems where you have found values > 0 are ZFS, the the RAID+LVM+FS implementation and GFS, a cluster filesystem. Both may have to store metadata to maintain file integrity that is not stored in inodes. It could be that ls -s counts in data blocks allocated for this metadata.

You can find that information on The Open Group's website, namely in the Register of Certified Products page.

Yes, Oracle Solaris is listed there.

You should check out this question: Is Linux a Unix? for some interesting info on what UNIX & Unix are, and their relation to Linux and other systems.