Most giant-types are like people. They look like people, they act like people (with some characteristic exaggerated), they like money, they fight and parley like us.
But trolls are not like people. They look like giant evil broccoli. They regenerate. They don't live in (relatively) nice places like castles, they don't throw boulders. They care not for the wealth of Men. They bite and scratch and claw and smother. They are fearsome, twisted nightmares.
Why is this? Why are most Giants like Men while trolls are so alien?