* cylinder filling is more efficient at maximum output request (on naturally aspirated engines operating in vacuum, due to less simple pumping loss, on turbocharged or supercharged engines, due to less wasted energy producing charge air and generally more optimized air/exhaust exchange). in short, air gets into the cylinder more efficiently in the first place at high torque requests

* combustion is more complete at high cylinder filling; not only does more air get into the cylinder, but it also burns more completely

* friction losses matter less at higher torque, especially on larger engines

Almost every ICE complexity advancement over the years is to hack around these basic properties: VVL and VVT to optimize pumping, direct injection, stratified injection, and dynamic compression hackery to get around cylinder filling efficiency, and cylinder deactivation / variable displacement to avoid friction and pumping loss from unnecessary engine size. but while ICE engines have made huge strides, they're still more efficient when they're loaded.

And note that this is efficiency, not absolute consumption - of course you'll consume more at higher load still, but the ratio is better. Combine this with the aerodynamics of most ICE pickup trucks and you have a scenario where ICE trucks frequently lose very little fuel economy / actual consumption when towing a trailer.

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.