That snippet doesn't check any return values so it is possible that the seek() failed. But I really suspect that VMS creates a sparse file and reports the amount of space the file is using, not the maximum byte offset for the file.

The same thing often happens under Unix -- the file isn't really preextended as far as allocating that much space on the disk. It is just that "ls" reports the maximum byte offset for the file, not how much disk space has been allocated to it.