All else being equal, a bulkier helmet is going to be more effective at absorbing impacts simply because the thickness of the helmet is all the room there is to provide a slower deceleration than your skull hitting the ground.

However, the worst brain injuries cyclists generally suffer are from the brain rotating inside the skull, something the bulk of a helmet actually makes it easier to cause.

That's why I don't wear one, aside from reserving the right to judge my own risks for myself. Also, mandatory helmet laws encourage the bogus idea that cycling is dangerous, discouraging cycle use and making cycling less safe for everyone because one of the best ways to make it safer is to get more people on bikes.
http://www.camdencyclists.org.uk/inf...um/hillman1991