Of course its not an absolute rule, just a general guideline. large creatures can have big heads and small creatures can have small heads, but unless your a really talented artist its hard to make it look convincing. Its also not just reliant on the head size but the rest of the characters body that conveys the size and weight. your character for instance has a big head but relatively small and meak body, where the lord of the rings troll has a large head but also a large and hefty frame to support it.
Heres a few examples to give you an idea of what I mean: