Cache Form table size is enormous
The {cache_form} table is a little funny, and behaves in a slightly different manner than other cache tables.

If you take a look at drupal_flush_all_caches() you will see that {cache_form} isn't cleared.  This is to protect in-progress forms from being nuked.

The system_cron() function does take care of pruning out old data from {cache_form} along with the other cache tables.

You really should run cron on all Drupal sites.  If you {cache_form} table is ginourmous, then I bet your {watchdog} and {session} tables are, too.  Many other modules run housekeeping activity as part of their own hook_cron() functions.

You may also want to poke around the issue queue.  There have been some bugs with {cache_form}, and you may be running into one.