Why can a small change in grid voltage control a much larger anode current?

In an electron tube, the control grid acts as a gate that regulates the flow of electrons between the cathode and the anode. Because the grid is positioned between these two electrodes, the electric field it creates has a significant influence on the number of electrons that can reach the anode.123

A small change in the voltage applied to the grid effectively modulates this electric field, which in turn controls the stream of electrons moving toward the plate. Since the grid is typically a wire mesh that intercepts very little current itself, it can exert this control over the much larger current flowing to the anode with minimal energy input. This ability to use a small input signal to regulate a larger output current is the fundamental principle behind the tube's function as an amplifier. The efficiency of this control is measured by transconductance, which represents the ratio of the change in anode current to the change in grid voltage that caused it.234