You missed skippys point. I almost did too. If you have exactly the same engine in two very different aircraft (say a Zenith 701 and a Sling, both with Rotax 912uls), then if one aircraft is just much more streamlined it will go faster for the same fuel consumption per hour. Then if it's going faster for the same fuel consumption, you are flying for a shorter period, so total consumption is lower for the faster plane.