Decrease file size upload in Media

That number is taken from wp_max_upload_size(), and there is a filter: 'upload_size_limit'. See wp-admin/includes/template.php.

So this should work (not tested):

add_filter( 'upload_size_limit', 'wpse_70754_change_upload_size' );

function wpse_70754_change_upload_size()
{
    return 1000 * 1024;
}