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In a previous work [Phys. Rev. B 85, 214408 (2012)], the conductance and the tunneling magnetoresistance in discontinuous high anisotropy magnetic tunnel junctions was experimentally studied. Different conduction regimes (sequential tunneling, co-tunneling, Kondo effect, and direct tunneling) and gradual transitions between them were found as a function of the temperature and the size of the clusters within the barrier. A simple theoretical model was suggested able to account for the experimental results even assuming no dispersion of the distribution of the size of the clusters. Here, we study, within this theoretical framework, the effect of the volume of the clusters within the barrier, and the effect of the thickness of the insulating barrier (or the tunnel resistance), on the transition between Kondo effect and co-tunneling regime. Clarifying the role of both parameters is of importance to understand and experimentally control the transition between the different conduction regimes. [ABSTRACT FROM AUTHOR]