Interesting experiment but there are too many variables left unattended to come up with a reliable answer. For starters in the video the safety is just roaring so energy is being wasted. The locomotive is most efficient if pressure is kept just below the pop off. The the out side temperature greatly affects these little locomotives. So as the day heats up the engine becomes more efficient in that it takes less fuel to boil the same amount of water. Anyway, I would think the answer would be the exact opposite. If you have a large locomotive, it is still burning fuel and having to haul it's own weight even with a light load. So the heavier the load up to the slipping point the more efficient it becomes as long as you aren't wasting steam by having the safety constantly popping off. You are after all, boiling the same amount of water so you are putting the same amount of steam through the cylinders to haul each load. As the load increases you have to increase the throttle, but these locomotives don't have cut off so the valve timing is the same no matter what you do. Interesting to see what Henry comes up with.