Empirical processes with bounded \psi_1 diameter

We study the empirical process indexed by F^2={f^2 : f \in F}, where F is a class of mean-zero functions on a probability space. We present a sharp bound on the supremum of that process which depends on the \psi_1 diameter of the class F (rather than on the \psi_2 one) and on the complexity parameter \gamma_2(F,\psi_2). In addition, we present optimal bounds on the random diameters \sup_{f \in F} \max_{|I|=m} (\sum_{i \in I} f^2(X_i))^{1/2} using the same parameters. As applications, we extend several well known results in Asymptotic Geometric Analysis to any isotropic, log-concave ensemble on R^n.