Burst mode in synchronous buck converter with current mode control?
However I just cannot understand how the light load decreases the load current


Light load is defined as low load current.  High load resistance is just one way that may happen.


  and therefore the inductor current, as mentioned in the link.


The inductor current is controlled by the converter circuitry; the low inductor current is not a direct effect of the light load; it is an effect of the controller reducing the duty cycle.


  As I know Vload=Rload∗Iload and Vload=D∗Vin. So if we have low load and highest input voltage, this implies that duty cycle is small and the current is going high


Again, Rload is not necessarily an accurate model.  Low load means that the load current is low, by whatever means that may happen.  When the load current is low, the load voltage goes up, and the controller reduces the duty cycle.


  What I am missing?


I'm not sure, but I suspect you're not taking the behavior of the controller into account.