The rapid isentropic compression of an initially uniform sphere of an ideal gas is compared with the homogeneous compression of a thin shell. A simple ablation model, in which the temperature of the ablated material remains constant, is incorporated which enables the properties of ablation-driven compression for these two cases to be compared. Expressions for power input, energy per unit mass, ablation efficiency and mass fraction compressed are all derived. Finally, the more complex self-regulating ablation model is used to illustrate the effect of a varying ablation temperature.