Small cars must be engineered properly to retain their safety credentials, but it's not impossible. Remember that all cars go through the same safety testing, regardless of their size. Crumple zones, airbags (you might want to make sure your Yaris has them), anti-lock brakes, stability control and the like all do their jobs and are standard on nearly every car sold in the States. Plus, as many drivers make the switch from big to small in the name of economy, the chances of meeting a nearly immovable object on the highway will only be reduced. Of course, we wouldn't recommend trying it for yourself or anything.
[Source: gas2.0 via Instapundit]