Why do you get more module cache files in version 1.5.11?
From Joomla! Documentation
Since Joomla! 1.5.11 the modules are cached for every page, so lots of cached files are created for the menu items with lots of pages.
To revert this back to previous caching behavior you need to edit /libraries/joomla/document/html/renderer/module.php and change the line
$contents = $cache->get( array('JModuleHelper', 'renderModule'), array( $module, $params ), $module->id. $user->get('aid', 0).md5(JRequest::getURI()) );
to
$contents = $cache->get( array('JModuleHelper', 'renderModule'), array( $module, $params ), $module->id. $user->get('aid', 0) );