As far as calories go, exercise doesn't have much impact. And maybe the Hunter/Gatherers are more efficient in their metabolisms.

As far as exercise goes, maybe one standard deviation is enough to produce a brain effect. And maybe there is a lot of variation within modern populations, with many far enough below the average to have a significant effect.

I find it really hard to believe that any healthy Hunter/Gatherers sit around all day like so many moderns do.