Experiments in which the effective anode area of a multipole ion source is reduced, in order to enhance its ionization and gas efficiency, show that there is a minimum area below which efficient operation becomes impossible. This phenomena is analyzed in terms of a model based on particle balance equations and is shown to be closely related to the reversal in plasma potential with respect to anode potential. The theoretically predicted operation regime is shown to be in reasonable agreement with that found in a variety of multipole sources. Comparison is also made with the limitations set by cathode area.