In comparing different species, the complex behaviours of each species correlate with the brain size. In contrast, when members of the same species are compared, the size of brain and behaviour may not be particularly correlated, but complexity of different brain regions can be correlated with variation in intelligence or behaviour.
For example, all humans share the approximately same anatomy of the brain, still shows a great variation in brain size and level of intelligence.
However, variation in intelligence can be correlated with difference in particular brain regions among the individuals. Thus, comparing the brain size of individuals and general intelligence is not possible.