Temperature regulation aspects might rate more highly as an influencing factor than one would think. Large bodies (and probably the head in particular) would be more resistant to hypothermia, whereas small bodies would be more resistant to heat-stroke.
Regardless of why, animals definitely do become larger further north and the brain size seems to follow the body size quite closely without much impact on intelligence. I don't know if arctic animals are quite on the scaling line. They do seem a bit smarter.
(The point of this comment is just to disentangle theory from observation.)
Please remember to have no heroes or villains, but this just looks plain bad to be honest. I'm lowering my estimation of the quality of Stephen J. Gould's work in this area.
USA today:
Haha. Humans.
The paper itself: