Form cache table not clearing

This is probably not connected to auth cache. Its common drupal problem

https://www.drupal.org/node/1506196
https://www.drupal.org/node/2057073
....

There is even a module for that
https://www.drupal.org/project/optimizedb

Thanks for the report. Could you please dig a bit in your data and try to figure out if there is a particular form which is responsible for this behavior? The following script selects the 1000 most recent entries, extracts the form ids, and sums up how many times that particular form has been stored in the sample.

define('DRUPAL_ROOT', getcwd());

include_once DRUPAL_ROOT . '/includes/bootstrap.inc';
drupal_bootstrap(DRUPAL_BOOTSTRAP_FULL);

$q = db_select('cache_form')
  ->condition('cid', '%form_state%', 'NOT LIKE')
  ->fields('cache_form', array('cid', 'data'))
  ->orderBy('created', 'DESC')
  ->range(0, 1000);

$counts = array();
foreach ($q->execute()->fetchAllKeyed() as $cid => $data) {
  $form = unserialize($data);
  $counts[$form['#form_id']]++;
}
arsort($counts);
var_dump($counts);

Nice snippet :)