The size of the engine will become insignificant in the future, because the engine will not be directly connected to the powertrain.
Capacitive storage of the engines energy production will allow you to store 1000 horsepower-seconds of energy, to be applied to the wheels as needed.
Make a mental graph of the real time power demands of your vehicle, then understand you must have a sustained power capability to climb steep grades.
Its not the oversized engines that are the real problem. It's running them all the time, and running them inefficiently almost all the time. The engines most important quality is its BFSC regardless of its size or power.
The failure in current designs is the belief that you need to use the engine to drive the car, which costs you 50% of it's potential efficiency.
actually it seems several people are. Adding weight will ALWAYS hurt fuel economy some, but if you are improving other variables more then the weight increase contributes to fuel economy, then yes, you will have a net increase in gas mileage.
If you could add weight to increase fuel mileage, everyone would be placing concrete blocks in their car year round.
Also I believe basjoos car did gain much if any weight, he chopped off some significant metal, removed glass, and replaced it with aluminum, lexan and I think abs plastic, which is probably considerbly lighter.
For what it's worth, my drives with the wife and child on board are consistently about 5 mpg lower than when I'm solo. Low 60's solo, high 50's with them.
My MPG always goes down with passengers. I usually don't hypermile as well because then I have to constantly explain why I just shut the engine off, why I just shifted into Neutral, why did I just downshift in an automatic, why am I driving so slow...