The molecular dynamics of polystyrene (PS) single-chain nanoparticles (SCNPs) with variable molecular weights and irreversible intramolecular cross-linking densities was studied, by employing broadband dielectric spectroscopy, supplemented by calorimetry (differential scanning calorimetry, DSC), in the melt. Regarding segmental dynamics, the alpha relaxation process was found to be retarded for low and moderate cross-linker fractions (CrFs), in agreement with the shift of the corresponding calorimetric glass transitions. Intriguingly, our data indicate that the dynamical behavior of SCNPs is strongly affected by the macromolecular characteristic of the PS precursors, that is, molecular weight (M-n) and CrF, as a secondary, Arrhenius-like relaxation process, beta*, appears in SCNPs with CrFs larger than a critical value. Our measurements reveal a transition from a strongly heterogeneous structure in loose SCNPs to a homogeneously compact structure in rigid SCNPs with increasing CrF. We propose a dynamic "phase diagram" of loose, dense, or rigid SCNPs, depending on the CrF and M-n of the precursor.