Height absolutely matters because frontal area, along with Cd, affects drag directly. Drag is the primary force that needs to be overcome at higher speeds per the road load equation. Your opinion has nothing to do with it, it’s all just basic physics. You’re right though that fuel economy has been mostly increasing for decades, but that is in spite of vehicle largess, not because size is irrelevant. Imagine how much better off we’d be if folks didn’t commute in trucks for no reason at all. And a big yes to transit, biking, and human centered development.