The foam was much lighter, and much more brittle, than a single brick that is about a tenth the size of the chunk that fell off, and it hit a glancing blow. I don't think it's a valid comparison.
One thing of interest: XBob posted a loop of pictures that I would interpret as showing the object fluttering outward (and a second object appearing, then disappearing), then swinging back towards the shuttle. I would expect foam to be blown almost straight back, but perhaps ice would have given it the inertia to be carried along more. This would result in a strike of less energy, as the relative speed would be lower, but at a more perpendicular angle. Thus a smaller, but deeper impact.
Either there is lateral motion, or the object at one point is moving FORWARD relative to the motion of the Orbiter.