A microscopic multi-j shell calculation is performed to describe the structure of the ground states of the even Sm isotopes within the pairing plus quadrupole model and in the collective pair approximation. Agreement with the experimental data is found for both quadrupole deformation ..beta.. and for the diffusivity of the Fermi surface. The variation with the mass number of the structure of the proton and neutron collective pairs is discussed in connection with the increased role of the quadrupole-quadrupole interaction and of the Pauli principle.