This paper focuses on collective contests for commons and club goods. Our main objective is to examine in this context the effect of group size on its performance. The main results specify conditions for the existence (non-existence) of the group-size paradox, namely, the situation where a larger group is less (more) effective in pursuing its interest because of (despite) the combined effect of the incentives that result in the free-riding problem and the tragedy of the commons. The paper also explains under what circumstances there exists a bias towards excessive or inadequate winning group size. Finally, it examines the effect of restricted excludability of the commons good within the winning group on the relationship between group-size and its winning probability.
