Attachments filesize dimension 0

I have a problem with filesize.

with this code

while( $attachments->get() ) : ?>
<a class="attachment-url" target="_blank" href="<?php echo $attachments->url(); ?>"><h6><?php echo $attachments->field( 'title' ); ?></h6></a><p class="attachment-caption"><?php echo $attachments->field( 'caption' ); ?></p>
<p class="attachment-size">Dimensione: <?php echo $attachments->filesize(); ?></p>
<?php endwhile; ?>

the result is always “filesize 0”

other fields works perfectly. It’s a server problem?
ty