We develop a theoretical model to study investors' trading behavior in the presence of large investors' influence on a firm's equity. We show that, for a good stock, large investors tend to invest more heavily than smart small investors, even though they predict the same equity returns. However, the smart small investors obtain more welfare and thus perform as free riders in this economy. Moreover, the analyses are extended to firms with more than one influential large investor. We find that: (i) the large shareholders collude in trading, and each tends to invest more aggressively as other large shareholders do, and (ii) firms with sole ownership can outperform those with dispersed ownership, if the impact coefficient of the former case exceeds or coincides with the aggregated impact coefficients of the latter.