Biocompatible magnetic nanoparticles are interesting tracers for diagnostic imaging techniques, including magnetic resonance imaging and magnetic particle imaging (MPI). Here, we will present our studies of the physical and especially magnetic properties of dextran coated multicore magnetic iron oxide nanoparticles, with promising high MPI signals revealed by magnetic particle spectroscopy (MPS) measurements. The Nanomag-MIP particles with a hydrodynamic diameter of 106 nm show an increase of the MPS amplitude by a factor of about two at the 3rd harmonic, as compared to Resovist. In particular, the signal improves progressively with the order of the harmonic, a prerequisite for better spatial resolution. To understand this behavior, we investigated the samples using quasistatic magnetization measurements yielding bimodal size distributions for both systems, and magnetorelaxometry providing the mean effective anisotropy constant. The mean effective magnetic diameter of the dominant larger size mode is 19 nm with a dispersion parameter of σ = 0.3 for Nanomag-MIP, and 22 nm with σ = 0.25 for Resovist. However, about 80% of the magnetic nanoparticles of Nanomag-MIP belong to this larger size mode whereas in Resovist only 30% do. The remaining Resovist particles are in the range of 5 nm, and, in practice, do not contribute to the MPI signal.