This is mostly because there is no real small cap premium. (optimalmomentum.com)
Further empirical evidence suggests small cap premium is concentrated in few sectors / industries which emerge out of nowhere and become sunrise industries. (aswathdamodaran.blogspot.ru)
Further small cap premium would be expected to be significantly positive in bull markets and significantly negative in bear markets, in other words small cap effect is a function of investor sentiment (risk - on vs. risk off sentiment). (aswathdamodaran.blogspot.ru)