I think that readfile suffers from the maximum script execution time. The readfile is always completed even if it exceed the default 30 seconds limit, then the script is aborted.
Be warned that you can get very odd behaviour not only on large files, but also on small files if the user has a slow connection.
The best thing to do is to use
<?
set_time_limit(0);
?>
just before the readfile, to disable completely the watchdog if you intend to use the readfile call to tranfer a file to the user.