Finally I got around this.

You need to modify tinymce args passed to wp_editor function. WordPress have a argument `wp_autoresize_on` to allow the editor to be resized automatically.

So instead of these:

    'tinymce' => array(
             'toolbar1'=> 'bold,italic,underline,link,unlink,forecolor',
             'toolbar2' => '',
             'toolbar3' => ''
         )
you need to use this:

    'tinymce' => array(
			'autoresize_min_height' => 100,
			'wp_autoresize_on'      => true,
			'plugins'               => 'wpautoresize',
			'toolbar1'              => 'bold,italic,underline,link,unlink,forecolor',
			'toolbar2'              => '',
		),
There are two additional args in here, `autoresize_min_height`, you can set it to desired height, and second one is `'wp_autoresize_on'      => true,`.
Apart from that you need to pass additional parameter to load the tinymce plugin for auto resizing and i.e `'plugins'               => 'wpautoresize'` and the auto resizing works flawlessly.

With these changes, I'd suggest you add some other checks in your code. Like for the function:

    function xxx_textarea_save_fields_metabox($post_id, $post) {
      global $post_id;

      if ( !wp_verify_nonce( $_POST['xxx_textarea_noncename'], plugin_basename(__FILE__) )){return $post->ID;}  
      if ( !current_user_can( 'edit_post', $post->ID ))                                      {return $post->ID;}   
      if( $post->post_type == 'revision' )                                                 {return;$post->ID;}

      if($_POST['_xxx_textarea']) {$xxx_content = $_POST['_xxx_textarea'];}
      else{$xxx_content = '';};

      if(get_post_meta($post_id, '_xxx_textarea', FALSE))
      {update_post_meta($post_id, '_xxx_textarea', $xxx_content);} 
      else{ add_post_meta($post_id, '_xxx_textarea', $xxx_content);};             

      }
Make sure to add a check for empty $_POST, otherwise you'll get notices on post edit screen.
I've made the changes and formatted the code ( You should be doing that ), here is the whole code for adding metabox.

    add_action( 'add_meta_boxes', 'xxx_textarea_add_fields_metabox' );
    function xxx_textarea_add_fields_metabox() {
    	add_meta_box(
    		'xxx_textarea_metabox',
    		'Intro',
    		'xxx_textarea_show_fields_metabox',
    		'new_xxx',
    		'side',
    		'default'
    	);
    }
    
    function xxx_textarea_show_fields_metabox() {
    	global $post;
    	$content = get_post_meta( $post->ID, '_xxx_textarea', true );
    	//Loads the editor to allow adding fresh content if there is no content already
    	$content = empty( $content ) ? '' : $content;
    
    	$args = array(
    		'description_name' => 'xxx_textarea',
    		'teeny'            => true,
    		'quicktags'        => false,
    		'media_buttons'    => false,
    		'tinymce'          => array(
    			'autoresize_min_height' => 100,
    			'wp_autoresize_on'      => true,
    			'plugins'               => 'wpautoresize',
    			'toolbar1'              => 'bold,italic,underline,link,unlink,forecolor',
    			'toolbar2'              => '',
    		),
    	);
    	wp_editor( $content, '_xxx_textarea', $args );
    	wp_nonce_field( plugin_basename( __FILE__ ), 'xxx_textarea_noncename' );
    }

    add_action( 'save_post', 'xxx_textarea_save_fields_metabox', 1, 2 );
    function xxx_textarea_save_fields_metabox( $post_id, $post ) {
    	global $post_id;
    
    	//Avoids notice and warnings
    	if( empty( $_POST ) ) {
    	    return $post->ID;
        }
    	if ( !empty( $_POST['xxx_textarea_noncename'] ) && ! wp_verify_nonce( $_POST['xxx_textarea_noncename'], plugin_basename( __FILE__ ) ) ) {
    		return $post->ID;
    	}
    	if ( ! current_user_can( 'edit_post', $post->ID ) ) {
    		return $post->ID;
    	}
    	if ( $post->post_type == 'revision' ) {
    		return;
    		$post->ID;
    	}
    
    	if ( $_POST['_xxx_textarea'] ) {
    		$xxx_content = $_POST['_xxx_textarea'];
    	} else {
    		$xxx_content = '';
    	};
    
    	if ( get_post_meta( $post_id, '_xxx_textarea', false ) ) {
    		update_post_meta( $post_id, '_xxx_textarea', $xxx_content );
    	} else {
    		add_post_meta( $post_id, '_xxx_textarea', $xxx_content );
    	};
    }

That should do it.