There is a phenomenon where individuals with hydrocephalus, a condition where the brain cavity is filled with fluid, can still exhibit normal intelligence. I suppose the two broad explanations are that brain matter is either denser in these individuals, or that a brain can still compute well with less brain matter at least in some cases (possibly by sacrificing on some other not-easily-measured mental faculties)
"There's a young student at this university," says Lorber, "who has an IQ of 126, has gained a first-class honors degree in mathematics, and is socially completely normal. And yet the boy has virtually no brain."
"There's a young student at this university," says Lorber, "who has an IQ of 126, has gained a first-class honors degree in mathematics, and is socially completely normal. And yet the boy has virtually no brain."
http://www.psych.ufl.edu/~steh/PSB6099/brainnecessary.pdf