We present a detailed comparison of simulations of disordered melts of symmetric AB diblock copolymers to predictions of the renormalized one-loop (ROL) theory. The behaviors of the structure factor S(q) and of single-chain correlations are studied over a range of chain lengths (N = 16,..., 128) for two models: one with harshly repulsive pair interactions and another with very soft interactions. The ROL theory is shown to provide an excellent description of the dependence of S(q) on chain length and thermodynamic conditions for both models, even for very short chains, if we allow for the existence of a nonlinear dependence of the effective interaction parameter χe upon the strength of the AB repulsion. The decrease in peak wavenumber q* with increasing χe is shown to be unrelated to changes in single-chain correlations. Results for all quantities are consistent with the hypothesis that the ROL theory gives an exact description of the dominant O(NÌ...-1/2) corrections to RPA and random-walk predictions in the limit of infinite chain length N.