We investigate the effect of cation size in the dc activation energy needed for oxygen ion conductivity, Edc, in highly disordered pyrochlore-type ionic conductors A2B2O7. Twenty compositions of general formula Ln2Zr2-yTiyO7 (Ln = Y, Dy and Gd) and Gd2-yLayZr2O7, were prepared by mechanical milling and their electrical properties measured by using impedance spectroscopy at different temperatures. We also evaluate, by using Ngai’s Coupling Model, the effect of cation radii RA and RB, on the microscopic potential-energy barrier, Ea, that oxygen ions encounter when jumping into neighboring vacant sites. We find that for a fixed B-site cation radius RB, both activation energies decrease with increasing A-site cation size, RA, as a consequence of the increment in the unit cell volume. In contrast, and for a given RA size, the dc activation energy Edc of the Ln2Zr2-yTiyO7 series increases when the average RB size increases. The latter behavior is explained in terms of the enhanced interactions among mobile oxygen ions as the structural disorder increases when RB approaches RA.