What's the difference between a fast and slow metabolism?

I needed to explain to a student in my health class why her friend, despite eating garbage all day never put any weight on meanwhile she had to eat very healthy to stay the way she was. I explained that a metabolism is very much like gas mileage. A fast metabolism is like a Hummer. To drive say 50 miles, you may use about 5 gallons of gas, since it doesn't do a very good job at converting that gas into energy. A slow metabolism is like a Prius, that can go 50 miles with probably about a gallon, making good use of that one gallon of gas.

I said look at the bright side, if this was a million years ago, your friend would have died from starvation while you would be able to chug along with no problem whatsoever! A slow metabolism is a positive evolutionary trait of survival.