The reduction in size of both NiO and Mn oxide crystallites can be explained as most of Mn ions are getting deposited in grain boundaries of NiO, they are impeding its crystal growth. This is known as Zener pinning and is a technique to stabilize the nano crystallites. As most of the Mn ions have been consumed in this process, the few remaining form oxide and hence the size is also reduced. Mn ions are acting here as impurity or activator ions and the interaction between themselves is not significant at low concentrations. However, as the MnO2 concentration is increased to 5%, the Mn ions start interacting among themselves and face repulsion hence their accumulation at the grain boundaries of NiO is reduced. The percentage of Mn ions forming oxide is more compared to the ones which impede NiO grain growth. This causes an enhancement of both Mn Oxide and NiO crystallite size. The Mn ions should be preferably in +2 oxidation states to either replace Ni +2 or go to the grain boundary of NiO as the interaction between the Mn +2 will be less compared to that for the higher oxidation states. The tendency of oxide formation by the Mn ions increases with increase in doping concentration as the grain boundary regions gets saturated. Interestingly a small peak corresponding to a particle size of 3 nm is also observed at 39 O which may be the strained NiO crystallites.