Using the 511-511 keV gamma ray coincidence-based Doppler broadening measurements, it is found that the positron trapping sites in nanocrystalline ZnFe2O4 are surrounded by Zn2+ ions. This was inferred from the observation of a characteristic peak around 515.8 keV in the normalised tail of the Doppler broadened gamma ray spectrum of ZnFe2O4 and which exactly coincided with that of pure Zn. The intensity of the peak enhanced when the grain size is reduced from 7nm to 6nm, depicting an increased probability of annihilation by the 3d electrons of Zn2+. This is attributed to the inversion of the spinet structure. A fraction of the octahedral Fe3+ ions in the neighbourhood of the oxygen vacancies are replaced by Zn2+ ions and a concomitant lattice contraction brings in closer interaction between the trapped positrons and the core electrons of Zn2+ ions.