Why would vehicle weight have any bearing on a vehicles efficiency traveling at a constant speed on level terrain? If we’re going to compare boats to vehicles

Why would vehicle weight have any bearing on a vehicles efficiency traveling at a constant speed on level terrain? If we’re going to compare boats to vehicles I would think that it’s at least fair to say that the vehicles terrain must be equally flat as that of the boat. If I’m the engine pushing a large truck at 55 miles an hour across the highway, I have no sense of the vehicles weight.