Smartness between a chimpanzee and a human is pretty clearly observable. But smartness between two humans is more often difficult to detect. Imagine you are plopped down in the middle of Southern Africa with Albert Einstein at his most productive age. If you encounter a bushman who finds you food, water, and shelter you will be sure to consider him much smarter than Einstein who keeps babbling about God and dice.

Anyway, brain size is probably not a very useful proxy for problem-solving ability. More likely, the size and health of the neo-cortex is a better indicator as it probably correlates to higher number of synaptic connections. Even then, if we discover a correlation between a kind of smartness and brain physiology, I doubt it would be a good measure of how successful, productive, motivated, happy, wise, etc. the person will be. Sometimes a hardship with a good set of mentors and friends produces the best results.