If your having issues importing a large (more than 20MB) WordPress XML File (WXR) you can fix it by increasing the amount of memory PHP is allowed. Here’s how:

  1. Open up your wp-config.php file
  2. Add the following line:
define('WP_MEMORY_LIMIT', '96M');

The WP_MEMORY_LIMIT option allows you to specify the maximum amount of memory that can be consumed by PHP.