Ceramic varistors based on the ZnO-Bi 2 O 3 -Sb 2 O 3 system, are usually sintered at 1150-1200 o C in the presence of a Bi-rich liquid phase that forms at 700-800 o C. Bismuth is critical for the formation of electrically active grain boundaries, and the Bi-rich liquid phase plays a major role in the grain growth kinetics. Therefore, uncontrolled bismuth oxide vaporization during sintering can heavily deteriorate the electrical response of the varistor, and the area-volume ratio of the green compact becomes a relevant parameter, along with the sintering temperature and soaking time. In the present work, we have studied the microstructure and the current-voltage characteristics of ceramic varistors with different area-volume ratios and sintering cycles. We find that, depending on the sintering cycle, there exists a value for the area-volume ratio that leads to a strong deterioration of the varistor electrical properties.