This assumes a filter capacitor is in parallel with the load, to average the current flow, and make it easier to read the volt level.
The buck converter has different modes to deliver voltage at the load, depending on how the coil is made to charge and discharge.
1. When the load is in a middle range, the proportional equation holds (Vout=D*Vin). The graph of current through the coil is a triangle wave which is above zero through the entire power cycle (more or less), and which touches the zero line (more or less) at the end of the power cycle.
2. As you reduce load impedance, it raises the time constant on the coil. The coil continually circulates a small amount of current around the output loop. Now it is acting to some degree as a choke, and less as a buck converter.
3. As you raise load impedance, volt level tends to rise to the supply V. The entire circuit starts behaving similar to a half-wave power supply, rather than a buck converter. The coil becomes idle through a greater portion of the power cycle.
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After you raise or lower load impedance, the volt level changes immediately, however you must allow several cycles for it to stabilize.
After you raise load impedance, the output voltage soars for a brief time, then settles downward to a stable level.
After reducing load impedance, the volt level drops for a time, then rises upward to a stable level.