In the 'How to Attack' section, the core book says "Regardless of attack throw and AC, all attacks on sleeping, paralyzed, or otherwise helpless targets automatically hit. If the attacker is not engaged by any other opponents, the helpless target can be automatically slain". The sleep spell has similar language.
However, under 'Sleeping Dragons' (Core pg 165), it says a sleeping dragon may be attacked at +2 to hit, thieves may backstab, etc. This seems to imply that the automatic hit and coup-de-grace does not apply to sleeping dragons.
If dragons are meant to be an exception to the rule, then should I apply a similar exception to other gigantic monsters? Is there a set of criteria that excepts a creature from being automatically slain while helpless?