In this work we seek to better understand the mechanisms governing pulse voltage trimming in disordered conductor-insulator composites. Therefore, we investigate the effect of the composition of thick-film resistors (TFRs) on sensitivity to high voltage pulses. We investigate four series of RuO2-based TFRs: two different glass compositions and two different RuO2 grain sizes. For each combination of glass and grain size, different RuO2 concentrations are studied. It is thought that the grain size influences the sensitivity of a TFR to voltage-trimming, but we show here that the volume fraction of conducting phase relative to its critical concentration is the essential factor governing trimming sensitivity. We also study the effect of the firing temperature on the sensitivity to voltage trimming and show that it decreases with increasing firing temperature.