Magnetization of antiferromagnetic nanoparticles is known to generally scale up inversely to their diameter (d) according to Néel's model. Here we report a deviation from this conventional linear 1/d dependence, altered significantly by the microstrain, in Ca and Ti substituted BiFeO3 nanoparticles. Magnetic properties of microstrain-controlled Bi1-xCaxFe1-yTiyO3-δ (y = 0 and x = y) nanoparticles are analyzed as a function of their size ranging from 18 nm to 200 nm. A complex interdependence of doping concentration (x or y), annealing temperature (T), microstrain (ϵ) and particle size (d) is established. X-ray diffraction studies reveal a linear variation of microstrain with inverse particle size, 1/d nm-1 (i.e. ϵ • d = 16.5 nm•%). A rapid increase in the saturation magnetization below a critical size d c ∼ 35 nm, exhibiting a (1/d)α (α ≈ 2.6) dependence, is attributed to the influence of microstrain. We propose an empirical formula M (1/d)ϵ β (β ≈ 1.6) to highlight the contributions from both the size and microstrain towards the total magnetization in the doped systems. The magnetization observed in nanoparticles is thus, a result of the competing magnetic contribution from the terminated spin cycloid on the surface and counteracting microstrain present at a given size. © 2017 IOP Publishing Ltd.