In the context of an infinitely repeated capacity-constrained price game, we endogenize the composition of a cartel when firms are heterogeneous in their capacities. When firms are sufficiently patient, there exists a stable cartel involving the largest firms. A firm with sufficiently small capacity is not a member of any stable cartel. When a cartel is not all-inclusive, colluding firms set a price that serves as an umbrella with non-cartel members pricing below it and producing at capacity. Contrary to previous work, our results suggest that the most severe coordinated effects may come from mergers involving moderate-sized firms, rather than the largest or smallest firms.