The standard deviation as one measure of performance spread shows that as you move down in size the range of performance gets drastically larger. One standard deviation on the downside will wipe out the average return for all categories. However, performance does not seem to be normally distributed. There is skew to the upside. More importantly, the range between the high and low manager within a category is multiples larger than the average.
The advantage of buying a large manager is with the clustering effect with performance. The cost of being wrong is minimized. So investors who pick from the large managers may not asking for something better but something that is less likely to be out of step with the rest of the group. Call it safety-first and performance second. Going down is size allows for better due diligence to be rewarded. It is a trade-off.