Intelligence is mostly attributable to environmental components and physiological components.

Environmental: exposure to stimuli, access to resources, how much lead you were exposed to as a child, and your network of family, friends, and the community who helped you gain your intelligence.

Physiological: the number of neural connections your brain has, the speed in which you can recall info, the speed/efficiency in which you can retain info, your ability to apply the information that you have to other scenarios, your ability to detect patterns, the speed in which your neurons can pass info to each other, the number of teratogens you were exposed to in utero, the intelligence of your parents – although that is probably more an environmental component than a biological one.

Anyway, there are numerous components that affect intelligence, but the size of the brain isn't really one of them. There are notable individuals who are considered above average intelligence and who were also born with only one hemisphere of their brain. Size doesn't matter much just like the size of a car doesn't predict how fast it will run.