Stoichiometric defects in silver bromide
Fundamentally, although ionic radii may offer some guidance as to defect formation, the bottom line is that the silver halides have complex bonding energetics that have to be taken into consideration.  I might suggest 'Defect structures in the silver halides', D.J. Wilson et al., Phys. Rev. B77, 064115 (2008) (presuming you have APS journal access). There the authors present DFT calculations on the formation energies of both Frenkel and Schottky defects in AgCl and AgBr.