Shear size isn't what makes SUV's safer than a smart car, actually some could make the argument that some SUVs are not safer due to an increased chance of roll over. It's the build materials and overall design that make SUVs safer in a collision situation. If you had say, a GMC Yukon with a thin flimsy frame it would probably produce the same results in the vehicle vs semi truck scenario.
The smart car satisfies gas mileage, but not the safety for this reason:
Currently a lot of hybrid and fuel efficient cars have been sacrificing heavy duty frame materials in favor of lighter, less strong ones for gas mileage due to the lessened weight.
SUVs on the other hand use strong, robust, frames with powerful engines in order to achieve good to excellent performance and cargo capacity. Sadly, the SUV has become the victim of the marketing pitch which is unfortunatly a catch-22: "The roads of today are very dangerous, you need an SUV to survive." This causes more SUVs to be bought, except it is all the SUVs that make the roads more dangerous.