Go back to Neurology 101. The brain doesn't work like a fucking engine, size is actually only important when you get down to about 5 pounds, where there simply isn't enough space for the type of organs creatures need for higher learning.
Connections are far more important. IQ is just a general(and heavily flawed) way of testing how fast your neurological connections are, and even then most of the human brain is for shit like memory and hormones anyways.
Sperm Whales have 20lbs brains. I don't see them attacking whaling ships en masse, do you?