The fragmentation processes of a medium mass system and of a very massive one formed in central collisions are compared within the Landau-Vlasov model taking into account both the isospin dependence and the two-body residual interactions. The simulations predict the formation of a roughly ellipsoidal source in the central Pb + Ag reactions while, for the Pb + Au system, the fragmentation occurs from an hollow source, the configuration of which being intermediate between bubble-like and toroidal shapes. This different shapes explain and allow to reproduce semi-quantitatively the two different profiles of the experimental kinetic energy spectra.