B C Z
They are supposed to be vaults in the ground. Making them thick looks weird. Grey to replace the Thief vault tile which is grey.
I always figured they were difficult to open chambers, and having them be the same approximate size as the treasure chests was to make sure people could see them.
Make them too thin and they look like cardboard.
If, however, they were "padlock" shaped instead of box-like, that might make sense.