I believe you're mistaken. Askenazi Jews score higher averages than whites in every IQ sub-category. The fact is that they have substantially higher verbal abilities than visuospatial (120 and 105, respectively). Because verbal ability (important for doctors, scientists, professors, lawyers, business, etc, etc) is the main predictor of economic success in developed civilization, it's much more important to have high verbal.
Also, average brain size in a population correlates about .4 - .5 to IQ. So although notable, it's definitely not the sole determinant.