show preview and not all page/forms
If you dont like what Drupal is doing by default for this preview feature, override it in your site theme, there is an example of this in the node_preview Drupal 7 API node_preview() comments.

the comment incorrectly titles the function THEME_Node_preview() it should all be lowercase

  /**
   *Output customized node preview on node edit and add forms.
   *
   * @return
   * This just outputs the Full node ignoring the teaser mode.
   */
  function YOURTHEME_node_preview($variables)
  {
      $node = $variables['node'];
      $elements = node_view($node, 'full');
      $full = drupal_render($elements);
      $output = '&lt;div class="preview"&gt;';
      $output .= '&lt;h3 class="post-preview" &gt;' . t('Preview of your posting') . '&lt;/h3&gt;';
      $output .= $full;
      $output .= "&lt;/div&gt;\n";
      return $output;
  }


If that still doesn't look right you could try altering this code further to:


provide an override to node.tpl.php or node--preview.tpl.php using theme suggestions for drupal's theming layer functions.
alter the results of $full above using Regular Expressions to hack off the beginning and end of the HTML page content.