The image below tells you the whole story. The mega-cap names that have a larger effect on the SP500 have outperformed their little brothers on a 1, 3, 6, and 12-month basis. The lower cap names are all lower in every time-frame.
According to Finviz, there's 21 mega-cap names, 571 large caps, 1041 Mid caps, 1621 Small caps, 1229 micro caps, and 599 nano caps. When you take the breadth of the entire market and plotted against the SP500 you more than likely will see divergences. It's not fair to compare 4,490 stocks not in the SP500 to the SP500.
Maybe it's not too late get reacquainted with the smart money index of the 90's, $OEX. The index is composed of the largest U.S. companies in the world.