> But you say, "As you decrease the amplitude of the PWM...";
> what do you mean by that?  The amplitude is the voltage, correct?

Yes, the voltage swing of the PWM signal about the 2.5V centre line

> Or is it the average voltage?

The average in this case is always 2.5V, as per Robert Ussery's
expanation

If you fed the PWM into some big-ass capacitor the peaks and
troughs would disappear to leave a DC voltage that reflected
the shape of the wave. A 50% PWM is half-above and half-below
the 2.5V line, so it averages to that 2.5V. A 10% PWM is much
more below the 2.5V line, so it averages to less than 2.5V, and
similarly with 90% averaging to above 2.5V (eg 100% PWM is 5V)

This is exactly the principle in a power supply. The sine wave
coming from the transformer goes through a bridge rectifier,
the output of which could be said to be comparable to your
PWM signal. The reservoir or smoothing capacitor is there to
average or integrate it to DC

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