xB fuel efficiency

Hi, I have an important question for anyone who knows a little about this car. Does anyone know how the Scion's design work? I consider it fuel efficient, it gets more mileage than many other cars of its weight. Can someone explain to me how it's shape or design will impact its fuel efficiency?

Thanks

im actually quiet dissapointed at the fuel efficiency.. it has 108hp (which is not enough to get you around if u constantly drive people) and it gets 31, while the corolla has 130hp (i know it is not a huge leap but it works) and added weight, still gets 29mpg, i think i would be better to buy a corolla instead of the xb