Monodisperse cubic spinet iron oxide magnetic nanoparticles with variable sizes were prepared following a multi-injection seeded-growth approach. As expected from such a well-known synthetic route, all samples were characterized by narrow size distributions, and showed excellent stability in both organic and aqueous media without the presence of aggregates, thus becoming ideal candidates for the study of their hyperthermia performance. Specific Loss Power measurements indicated low heating powers for all samples without a maximum for any specific size, contrary to what theory predicts. The magnetic study showed the formation of size-dependent nonsaturated magnetic regions, which enlarged with the particle size, evidencing a clear discrepancy between the crystal size and the effective magnetic volume. Strain map analysis of high resolution transmission electron micrographs indicated the presence of highly strained crystal areas even if nanoparticles were monocrystalline. The origin of the crystal strain was found to be strictly correlated with the seeded-growth synthetic procedure used for the preparation of the nanoparticles, which turned out to alter their magnetic structure by creating antiphase boundaries. Considering the calculated effective magnetic volumes and their magnetic dispersions in each sample, a reasonable agreement between hyperthermia experiments and theory was obtained.