Is this an OS thing

No. It shows the current size of the file, whether it's been committed to disk or not.

The writer is probably buffering its output.

>perl -MIO::Handle -MFile::stat -le"open $fh, '>tmp'; print $fh 'foo';
+ for (1..2) { print stat('tmp')->size; $fh->flush }"
0
5
[download]

is there any way for me to determine if the file is completely there or not

Sorry, it's impossible to predict whether someone will write to a file.

A common practice is to have the file creator build the file in one directory, then rename it into a different directory. The reader simply checks for the presence of files in the later directory.