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Think of a capacitor connect between the gate and the drain. If the voltage changes by vi on the gate the voltage changes on the drain by -G(gain) X vi. So the voltage across the capacitor has changed by (G + !) X vi. So looking in from the gate the capacitance has increased by G+1. So as the gain is changed the input capacitance changes.
In the cascode amp, the voltage on the drain changes very little as its input impedance is 1/Gm (~200 ohms?), so the input capacity of the first transistors gate will be twice Cg-d. The AC component now modifies the G-D voltage of the top FET, so it amplifies as usual. It will have no Miller effect as the gate is decoupled to earth, so the Cg-d, just appears across the load resistor.