The undesirable area ratio of a small anode : large cathode can arise where a paint coating has been applied to the anode only and this coating has discontinuities (defects, holidays).

Unfavorable area ratio -schematic
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Clearly, such a galvanic corrosion situation is undesirable (full coverage of the electrodes by corrosive electrolyte is assumed). Coating the cathode only, or coating both the anode and the cathode, is generally preferable to avoid the unfavorable area ratio effect in a galvanic corrosion situation.

References/Literature:

REMR Technical Note EM-PC-1.1: Forms and Causes of Galvanic Corrosion in the Coastal Environment.

Links:
Galvanic corrosion - introduction

Galvanic corrosion example, history