In a $P$-$V$ indicator diagram, the magnitude of work done on a gas during compression is given by the area under the $P$-$V$ curve. For compression from a given initial state to half its initial volume, the adiabatic curve is steeper and lies above the isothermal and isobaric curves. Therefore, the area under the adiabatic curve is the maximum among them, which implies that the maximum work is done on the gas during the adiabatic compression. In an isochoric process, the volume remains constant, so the work done is zero.