another thing u got to understand is the cars today are heavier and have more crap in them like airbags, abs, etc.... to meet crash and saftie requirements. there also got more stuff in them for emisions (small things [upto 6 o2's as aposed to one, and that wiring adds up.]) strip the car down to the esentals like lets say the LX 91 civic did and u'd get the same milage.
it is true that they design these cars to have some zip and fe suffers because of it,
i read that toyota rearranged the gearing with the yaris (compared to gen1 and echo) to make it quicker,
the yaris also has a drive-by-wire system and electronic steering, this apparently gives it small fe improvements, but probably don't compensate for the gearing issues.
toyota offers 4 different yaris engines in europe from a 1.0L petrol to a 1.4L diesel, apparently in n.america, we want power, which is kind of ridiculous even with a 1.5L engine. if you want power- don't get a yaris!