Title's a bit misleading. The study actually found that mothers with a low waste-hip ratio had smarter kids. That translates to big hips (and narrow waist), not the image of rolls of fat and a gigantic gut that normally come to mind when you say "fat moms".

Also, I'm curious if they actually measured polyunsaturated fat levels and correlated them. I've heard other studies that suggested big hips lead to healthier, smarter children because they lead to easier childbirths and inflict less trauma on the fetus during delivery. They could control for this by testing a group of kids delivered by caesarean.