You probably have Debian or Ubuntu running on which PHP has an issue with session disposal. Your ezsession table is grown big so try giving to cleanup script more time (with -d set_time_limit). If it doesn't help I would try to cleanup all session and put your expired_session script in cronjob every night.
Don't know about this tweak but if you put cleanup of expired_sessions in cronjob every night your ezsession table should not grow (it size will depend on number of site users and session timeout settings)
