Buck Converter with RLE Load
You can't figure out how long your switch is going to have to be on without knowing what current the load draws!

Remember, the physical idea of your converter is "charge the field in an inductor, exactly as much as necessary to supply the right output voltage". 

A load takes energy from that field, so you'll have to replenish that, and that's exactly what you do during your on time. So, if less energy is taken, then there's less on-time.