Experiments on a small tokamak, TOSCA, have shown that a hexapole magnetic field produces axisymmetric jumps in major radius. These occur when the hexapole field is used to produce an outward pointing triangular deformation to the plasma cross-section. A theoretical model is presented which interprets these jumps as the transition between two equilibrium states following a perturbation to the plasma parameters, for example, the self-inductance per unit length and poloidal beta. The model predicts the magnitude of the jump as a function of the hexapole field and the initial plasma position.