Cleon
Legend
Yes, let's just say it only drops rocks of up to PC weight, so the damage is just normal falling damage. That can still get pretty high, but we can make it a ranged attack with a smallish range increment to make it hard to hit.
But that would mean it'd be pretty pointless for the Roc to use that attack. It could do 8d6+36 damage with a diving Flyby attack AND get to add Power Attack to it for the same damage and the option of Snatching its prey.
If the boulder doesn't do similar or greater damage, why would it bother?
Not to mention a Great Roc would look like it's tossing pebbles if it drops 200 lb stones.