I think... it's the question of whats gonna give? A large thin washer or a solid piece of cast metal (head). The washer being unevenly supported will bend quicker before it wears out a solid thick piece of metal. The The larger washers didn't cause any more indentations, instead the larger washer distorted sinking its inner part into the empty area of the indentation made by the smaller washer that used to be there. As the washer distorted the clamping force of the bolts decreased, causing the head to lift.