When a mercury thermometer uses a larger bulb with the same capillary tube, the volume of mercury increases. Since the capillary cross-section remains constant, the same temperature change causes a greater length change in the capillary. This means the thermometer can detect smaller temperature differences, so its sensitivity increases.

Mathematically, sensitivity $S\propto \frac{\mathrm{\Delta V}}{A}$, where ΔV is the volume change and A is the capillary area. A larger bulb gives a larger ΔV for the same ΔT, increasing S.

Final answer: sensitivity