From understanding never density in mammals is mostly the same. So large body mass means more nerves that have to communicate with central nervous system. Also

From understanding never density in mammals is mostly the same. So large body mass means more nerves that have to communicate with central nervous system. Also some people have argued that fish can't feel pain or are less sensitive to due to their lower nerve density from my understanding. A whale is a mammal so I would assume it's going to have similar nerve density. Which means the brain has to be able to take input from a larger volume of body so that brain in large animals would make sense to be larger.