Really, the physics of crumple zones is not rocket science. The stiffness of the zone determines the amount of force applied to occupants while the depth of the zone (for a given stiffness) determines the maximum impact speed the zone can absorb. Short zones work just fine, but become ineffective at lower speeds than longer zones.
IIRC there is considerable disagreement on how much deceleration a human body can safely withstand, and air bags complicate that. The major concern is aortic rupture, and air bags alone have been known to cause aortic rupture at speeds as low as 10 mph. It still doesn't change the shape of things - longer crumple distances are effective to higher speeds than shorter ones. Where the cutoff is for the SMART car is something I couldn't find.
Mike