Some studies have discovered that brain size matters for intellectual ability and having bigger brain means having better intellectual ability. The most interesting study that confirmed this took part in 2005 McMaster University researchers have found that brain size really matters. They also found that there are differences between women and men when it comes down to brain size. In women, verbal intelligence is correlated with brain size, accounting for 36 percent of the verbal IQ score. In men, this was true for right-handers only, indicating that brain asymmetry is a factor in men.
But on the other hand lets look at Albert Einstein's brain. Albert Einstein is undisputedly one of the biggest geniuses in our history but his brain was of average size. However his brain did have unusual brain anatomy and many scientist believe this is the main reason why Einstein had different flow of thoughts compared to all other. In fact one part of the Einstein's brain was missing and according to some scientists this enabled neurons in this part of his brain to communicate better resulting in one of the greatest mind in human history.
Lets go a little bit to animal world to see size of their brains. For instance the average adult human brain weighs around 3 pounds (1,361 grams) which is close to average brain weight of dolphin. We consider dolphin to be very intelligent animals, right? Well how about sperm whale, nobody considers this whale to be genius among animals and yet it has brain that weighs around 17 pounds (7,800 grams). We consider dogs to be smart but they only have average brain of around 2.5 ounces (72 grams). Of course we should also take into account the size of the total body, and it is not the same to compare brain of sperm whale that weighs more than ten tons to brain of dolphin that weighs only around 350 pounds (158.8 kilograms). Having bigger body usually means having larger skull, and therefore having bigger brain.
What we should focus here is brain to weight ratio, if we focus on this then we'll see that we are the smartest ones on planet since our brain to weight ratio is about 1-to-50. For most other mammals, it's around 1-to-180, and for birds, it's around 1-to-220. As you can see the brain takes up more weight in a human than it does in other animals, and makes us smarter than animals.
So size does really matter when it comes to brain, but since we only know so little about brain who knows what else our brain is capable of. Perhaps size isn't that important for making us smart as we now think. Well, it's up to future to tell us more. For now the size does matter.