if raised against another surface or lowered on a floor or such, anything on/under it will get smashed. If there is no surface against which it is raised, it will launch people skywards. Doing this on a creature that is too large destroys the bridge
Actually, the presence or absence of walls around the raising bridge has no effect whatsoever on whether items will be smashed by it.
When a drawbridge is raised, one side of the drawbridge becomes a wall. Anything on those tiles which become a wall will be destroyed. This happens regardless of whether there are any other walls around the drawbridge or a roof over it. Creatures or objects on the rest of the bridge will be thrown up to 11 tiles in a random direction. If they can't move because there's a wall or roof in the way, they simply don't go anywhere.
If you try to raise a drawbridge with a creature of size greater than about 1,200,000 on it, the bridge simple won't raise. It won't be destroyed, it just won't raise.
When a drawbridge is destroyed, any creatures or objects in the way are destroyed. This happens regardless of whether there are walls around the drawbridge. If any creature of size greater than about 1,200,000 is in the way when you lower the drawbridge, the bridge will deconstruct into its raw materials instead of destroying the creature.