Buck converter efficiency Vs load current:
When there is no load connected there is no power output but, there will be a little bit of power drawn by the converter to keep it running. This scenario is therefore zero efficient. As output load power increases thus efficiency rises from zero and at some arbitrary point the converter reaches maximum efficiency. At greater loads efficiency decreases due to conduction losses in the switching transistor and inductor (for example).

Basically efficiency = power out / power in and for very low powers being taken by a load the efficiency is going to be very poor.