media_handle_sideload does not seem to be generating different size images.

I have the following code which uploads an image from a url and attaches it to a custom post type in a plugin I am developing. (edit: This code is located in my update_post function.) The image is being uploaded and attached to the post. However I am not seeing the different size images that I believe are supposed to be generated. I have looked into the upload folder and only the original image is there.

Am I doing something wrong or am I misunderstanding how this is supposed to work?

Code:

$tmp = download_url( $image_url );
 $file_array = array(
     'name' => $_POST['preview_filename'],
     'tmp_name' => $tmp,
 );
 // Check for download errors
 if ( is_wp_error( $tmp ) ) {
     @unlink( $file_array[ 'tmp_name' ] );
 } else{
     $id = media_handle_sideload( $file_array, $post->ID );
     // Check for handle sideload errors.
     if ( is_wp_error( $id ) ) {
         @unlink( $file_array['tmp_name'] );
     }
     else {
         update_post_meta($post->ID,'rfttc_attached_image_id', $id);
         update_post_meta($post->ID, 'rfttc_preview_filename', basename ( get_attached_file( $id ) ) );
     }
}