A comparative structure and stability study is performed on a sequence of (MgO)_n, (CaO)_n, (SrO)_n and (BaO)_n clusters, 4 ⩽ n ⩽ 12. The B3LYP functional is employed to seek preferred cluster growth pathways. Similar stabilities are found for two growth strategies, one producing rock-salt-like structures, and a second which employs stacks of hexagonal (MO)₃ rings. Structures based on hexagonal rings are slightly preferred in the case of MgO and BaO, while for CaO and SrO the rock salt structural analogue prevails. The shift in isomer stability order is discussed in terms of packing of spheres. The small magnitude of the atomic size difference for oxygen and the corresponding metal atoms in the cluster implies preference for the rock salt structure. Larger |ΔR_A| result in a preference for hexagonal ring-based structures.