Essentially, a big engine doing no work is more efficient than a smaller engine doing lots of work.
A big engine doing no work would be "off".
In a pair of hypothetical vehicles who's only difference is big vs small engine, the "work" requirement to move them down the road would be equal. Well... if we bend that "equal" a bit, the small engined version would have a bit less drivetrain loss due to the relatively smaller components.