Stepwise modulation of the ring size of metallacyclic compounds via subtle ring-constraint effects has been established. The reactions of (COD)PdCl2 with p- or m-psb ligand [COD = 1,5-cyclooctadiene; p-psb = 1,4-bis(dimethyl-4-pyridylsilyl)benzene; m-psb = 1,4-bis(dimethyl-3-pyridylsilyl)benzene] in a mixture of acetone and ethanol at room temperature produce the metallacyclodimers [PdCl2(p- or m-psb)](2). The cyclodimeric species of [PdCl2(p-psb)](2) at the boiling temperature or via the sonication of the chloroform solution is completely converted to the cyclotrimer [PdCl2(p-psb)](3). Direct reaction of (COD)PdCl2 with p-psb in a mixture of acetone and ethanol at reflux temperature yields the same trimer, [PdCl2(p-psb)](3). Furthermore, equilibria between the kinetic product, [PdCl2(p-psb)](2), and the thermodynamic product, [PdCl2(p-psb)](3), have been observed in NN-dimethylformamide as well as in dimethyl sulfoxide. In contrast, the cyclodimeric structure of [PdCl2(m-psb)](2) is retained under the same treatment conditions for 40 h; that is, the trimeric species, [PdCl2(m-psb)](3), is not formed. Such a notable difference between [PdCl2(p-psb)](2) and [PdCl2(m-psb)](2) might be explained by their different angle constraints.