BiFeO3 (BFO) nanoparticles (NPs) were studied using First-Order Reversal Curve (FORC) and temperature-dependent magnetometry measurements. The BFO NPs were fabricated by a sol–gel method, while the crystal structure and the average particle radius were obtained by powder X-ray diffraction analysis and Small-Angle X-Ray Scattering (SAXS) measurements, respectively. The NP size varies below and above the typical bulk BFO spin cycloid length (λ= 62 nm). Below λ, the NPs show ferromagnetic-like hysteresis loops where the saturation magnetization decreases while nanoparticle size rises. This magnetic behavior changes for NP size over λ, which only exhibits a paramagnetic contribution. The FORC distributions indicate the presence of two competing size-dependent contributions to the observed magnetic signal Also, the FORC distributions show that in the ferromagnetic regime there are two competing size-dependent contributions to the observed magnetic signal. Our results suggest the existence of a magnetic core–shell structure in NPs below λ, possibly driven by the strong spin–lattice coupling.