In order to show that the engines are wastefully large, you first have to show how a smaller engine provides better fuel efficiency over the 95% range, and then

In order to show that the engines are wastefully large, you first have to show how a smaller engine provides better fuel efficiency over the 95% range, and then show how the remaining 5% is unimportant. For example, if the big engine is 5% less efficient over the power range of the little engine, but the increased max power it provides allows you to accelerate out of getting stuck in the snow, or travel at safe speeds up mountain roads, reasonable people might decide that the trade offs are acceptable.