Stocks with a lower market value (mid and small caps) realize higher returns than those with a higher market value or capitalization (large caps).
In academic circles the discussion focuses on whether the better expected performance of companies with a lower market value is only compensation for the higher degree of risk.
Another point of discussion concerns the transaction costs that accompany a strategy based on this factor. These costs are higher for equities with a lower market value due to the reduced liquidity.
Robeco implicitely uses the Size factor by including mid and small cap stocks in its investable universe.