Why boost converter efficiency increases at low D and load resistance?
Current always flows through the inductor resistance. When the switch is in position 1, Vg only supplies power to RL and the inductor. When the switch is in position 2, Vg also supplies power to R. No energy is transferred from source to load in position 1, so position 1 is 0% efficient. Think about what happens with a 0% and a 100% duty cycle. 

RL isn't the load resistance, it's the inductor resistance. Decreasing the inductor resistance obviously decreases the conduction losses.