Comment by apelapan

Efficient in this situation means units of fuel per units of work.

Of course an ICE uses more fuel in absolute terms on a higher load than a lower one. But the proportion of fuel wasted compared to fuel turned into propulsion improves as load is increased (at least up to a point, maybe not all the way to absolute maximum load).

Electric systems also have efficiency curves, but they are less drastic within the primary load range.