We present ab initio calculations of the zero-temperature composition dependent spin transition pressures in rocksalt (B1) (Mg1-x,Fex)O. We predict that the spin transition pressure decreases with increasing Mg content, consistent with experimental results. At high-pressure, we find that the effective size of Mg is smaller than high-spin Fe but quite close to low-spin Fe, consistent with a simple compression argument for how Mg reduces the spin transition pressure. We also show that the spin transition is primarily driven by the volume difference between the high-spin and low-spin phases, rather than changes in the electronic structure with pressure. The volume contraction at the transition is found to depend non-monotonically on Fe content. For FeO we predict a B1 → iB8 transition at 63 GPa, consistent with previous results. However, we also predict an unexpected reverse transition of high-spin iB8 → low-spin B1 at approximately 400 GPa.