Huge cache_menu table
It has been a bug in Drupal, but should be corrected since Drupal version 7.13. See more details here: http://drupal.org/node/1234830

As a workaround, you can empty (truncate) the cache_menu table, since it is just a cache table and will be regenerated. Of cource, you'll have a trade-off in performance, in that case.