Dolphins have a funny brain to bodymatter ratio....look it up
It's true, they do have very large brains for their body size. I believe their ratio is somewhat better than our own, even.
HOWEVER:
The reason for this is that the portion of their brain which deals specifically with spatial navigation is far larger than ours.
This is also the reason that Eskimos have the largest brains of all human groups, and when whites first encountered them they marveled at their ability to trace their way back from hunting expeditions in the frozen expanse using landmarks the whites could not perceive.
Yet, Eskimos do not score the highest of any group on IQ tests. That would be Ashkenazi Jews and then East Asians next after them.
This is also why when Eurasians switched to an agricultural existence their total brain size actually DECREASED, the navigation/spatial center of the brain became less necessary in a more stationary environment, and shrunk accordingly (brains are incredibly resource-hungry and nature isn't keen on permitting unnecessary brain function to stick around)
Regardless, back to dolphins... without real culture and the ability to pass along massive amounts of learned information from previous generations as we can, and without the ability to manipulate their environment as we can, whatever intellectual potential they have is somewhat meaningless...
and intellectual ability does a back and forth with it's use, so the brain will enable certain behaviors, then those behaviors will in turn influence the brain... a very complex dance. As in, you start to be a species that makes tools and now natural selection has the ability to sort of get a grasp on that and select for it's increase or decrease.