Convective self-aggregation refers to a phenomenon in cloud-resolving simulations wherein the atmosphere spontaneously develops a circulation with a convecting moist patch and a nonconvecting dry patch. All previous studies have found a sharp transition to aggregated convection when the domain size exceeds a critical threshold, typically in the range of 200–300 km. Here, we show that cold pools are responsible for this sharp transition. When cold pools are inhibited, self-aggregation occurs at all domain sizes. In this case, the aggregation strength decreases smoothly as the domain size L is decreased below about 200–300 km. A streamfunction analysis reveals two distinct sources for the air subsiding into the dry-patch boundary layer: a moist, shallow circulation and a dry, deep circulation. The deep circulation scales with L, whereas the shallow circulation does not. At small L, the shallow circulation dominates, thereby weakening the aggregation.