The physical properties of perovskite manganites are critically governed by the A-site cationic composition; however, the respective roles of the average ionic radius <r \(_{A}>\) and the size variance \(\sigma ^{2}\) remain entangled. In this work, we systematically investigate the cooperative effects of <r \(_{A}>\) and \(\sigma ^{2}\) on the structural, magnetic, and transport properties of La \(_{0.8}\) Sr \(_{0.2}\) MnO \(_{3}\) (LSMO) by isovalent substitution of Sr with smaller Ca and larger Ba, yielding La \(_{0.8}\) Sr \(_{0.1}\) Ca \(_{0.1}\) MnO \(_{3}\) (LSCMO) and La \(_{0.8}\) Sr \(_{0.1}\) Ba \(_{0.1}\) MnO \(_{3}\) (LSBMO), respectively. Although the lattice parameters increase monotonically with <r \(_{A}>\) , the Curie temperature T \(_C\) does not follow the same trend; instead, LSBMO exhibits a lower T \(_C\) than LSMO despite possessing the largest <r \(_{A}>\) . This anomalous behavior reveals that the local lattice distortions exert a more dominant suppression on the double-exchange interaction than the concurrent bandwidth broadening. Transport measurements indicate that the high-temperature insulating regime follows the correlated small-polaron hopping model, whereas the low-temperature metallic conduction is governed by combined electron–electron and electron–phonon scattering mechanisms. LSMO exhibits the smallest polaron binding energy, and the highest metal-insulator transition temperature, highlighting the number of distinct A-site cation species—i.e., chemical complexity—as a third crucial parameter beyond <r \(_{A}>\) and \(\sigma ^{2}\) . These findings provide quantitative experimental evidence for the intricate interplay between structural disorder and physical functionalities in perovskite manganites.
