Mixtures of small amounts of short diblocks (cosurfactants) and chemically identical large diblocks (surfactants) are considered, when the large diblocks are in the strongly segregated regime. Due to the enthalpy gain, the short diblocks segregate to the interfaces and change the interfacial properties. The competition between the interfacial free energy reduction and the packing free energy change determines the phase behavior of the system. It is shown that large changes can be expected in the global phase diagram when both short and long chain composition ratios are varied. At some places on the phase boundaries between structures of different symmetries the increase in the packing free energy density corresponding to the addition of nearly symmetrical short diblocks cancels the reduction in the interfacial free energy density, and varying the concentration of small chains has no effect on the stability of the phases in the vicinity of these structural fixed points. Elsewhere, large changes in the stabilities of phases of different symmetries can occur depending on the composition ratios of the short and long copolymers, and whether the short molecules behave as compatibilizers at the domain boundaries or act more like homopolymer fillers in the bulk domains. Density profiles of both large and short diblocks, based on mean-field calculations, are presented.