Obviously, a large monster is less likely to suffer fromor itself. There is no way a huge Dragon is going to be scared of a Troll, for example.
A creature that causes Fear is not affected by enemies that cause Fear. Faced with an enemy that causes Terror, a Fear-causing monster only suffers Fear, not Terror.
A creature that causes Terror is not affected by Fear or Terror at all.
For example, a Troll causes Fear and a Dragon causes Terror. The Dragon is not at all worried by the Troll, but the Troll fears the Dragon. This special immunity also applies to any rider of a Fear- or Terror-causing monster (or steed), so a Dragon rider wouldn't be afraid of a creature that would frighten him were he on foot.
Also, Fear tests or Terror tests are passed automatically if the Fear- or Terror-causing enemy is currently fleeing - their scariness is greatly hampered by them attempting to run away from the battlefield as fast as their legs can carry them...