With whales in particular, they don't even have arms or legs, so I wouldn't expect them to have large regions of the brain devoted to, say, fine-motor skills.
Ah, but they also have a complete three dimensions to move in, unlike us humans who only have about 2.5 dimensions to move in. Also, they have a number of different "limbs": tail, multiple fins, etc.
Yet, it is apparent (to many people) that whales and elephants don't have drastically higher intelligence than humans. So their extra brain regions have to go somewhere. Where do they go to?
Why not? How do you accurately measure an intelligence that is structurally different from yours? You have to know how that intelligence works, and for that to happen you need to overcome the language barrier (which is still an issue with cetaceans, since many people think they're just dumb animals! Maybe some cetaceans are like primate apes. Maybe others are like primate humans!).
Here is a pdf overview of Cetacean brains.
I hope this answers your question:
A few parts pulled from the article:
Cetaceans have very large brains, yes, but humans hold the top of the proportion scale: our brains are larger in comparison to our body size than most other species.
Cetacean brains are, for the most part, like other brains. That is, they are there for the processing of cognition (among other things). A large part of this processing power goes to echolocation.
Regarding similarities between primate and cetacean brains:
the expansion of the insular and cingulate cortices in cetaceans is
consistent with high-level cognitive functions—such as attention,
judgment, intuition, and social awareness—known to be associated with
these regions in primates.
This view is further supported by the
observation that the anterior insular
and anterior cingulate cortex in
cetacean species having the largest
brains exhibit a large number of large
layer V spindle neurons, similar to those originally reported
to be unique to humans and great apes. These particular neurons are
considered to be responsible for neural networks subserving aspects of social
Furthermore, though specifically regarding dolphins:
Laboratory studies of bottlenose
dolphins have documented
various dimensions of their
intellectual abilities. These include
an understanding of ... (declarative knowledge); an
understanding of ... (procedural
knowledge); an understanding of ... (social knowledge); and
an understanding of ... (self
knowledge). All these
capabilities rest on a strong foundation
of memory; investigations have
demonstrated that bottlenose dolphin
auditory, visual, and spatial memory are
accurate and robust.
Field studies have documented
impressive cultural learning of dialects,
foraging sites, and foraging and
feeding strategies in cetaceans.
So, to recap, the cetacean brain lends itself to, among inumerable other things, advanced auditory (sensory), personal (self), and interpersonal processing (social). Also, dolphins, and maybe other species as well, have advanced memory and language processing capabilities.