Two reasons to be skeptical of any of the claims about BRAIN SIZE. (The abstract emphasizes other things like "global connectivity" so pelase don't read these comments as a destruction of TFA. More how Slashdot reported it.)
1. The history of intelligence testing. Read S. J. Gould's "The Mismeasure of Man" and you'll find good reason to immediately be skeptical of any research such at this.
2. The statistical assumptions. So what's on the left hand side of the model? Obviously, some measure of intelligence. Thus the right hand side variables (size of different regions, etc.) are predicting an intelligence composite. Which assumes that intelligence is something determined in part by those right hand side variables or something correlated with those variables. These studies assume an at least semi-valid measure of intelligence. If they want to claim a causal relationship, I want a damn good measure of intelligence. Otherwise they're finding connections between something that may well be correlated with whatever we call intelligence, and we have no reason to believe brain size.
Furthermore, to say 6% of the variance is explained by brain size, we're assuming there's nothing unmeasured that doesn't also correlated with brain size and the dependent variable (intelligence). Historically, this situation has often presented itself. For example, when the better nourished happened to be more intelligent AND have bigger brains because of their nourishment.
A final problem with ALL intelligence research that wants to claim a causal relationship is that intelligence isn't something we can really manipulate in the lab, so we're always going to have to worry about endogeneity and selection issues. we can't control "how people get assigned to intelligence," and so we always may have something unmeasured which really explains what we find (by way of correlating with, for example, "global connectivity.")