Amusing that you mention the Smart Fortwo.
http://www.iihs.org/news/rss/pr041409.html
ARLINGTON, VA — Three front-to-front crash tests, each
involving a microcar or minicar into a midsize model from
the same manufacturer, show how extra vehicle size and
weight enhance occupant protection in collisions. These
Insurance Institute for Highway Safety tests are about the
physics of car crashes, which dictate that very small cars
generally can't protect people in crashes as well as
bigger, heavier models.
"There are good reasons people buy minicars," says
Institute president Adrian Lund. "They're more affordable,
and they use less gas. But the safety trade-offs are clear
from our new tests. Equally clear are the implications when
it comes to fuel economy. If automakers downsize cars so
their fleets use less fuel, occupant safety will be
compromised. However, there are ways to serve fuel economy
and safety at the same time."
http://www-nrd.nhtsa.dot.gov/Pubs/808569.pdf Large vehicles have historically been more stable and
provided more protection for their own
occupants than small ones, but they presented a greater
hazard to other road users. Between 1985
and 1993, the population of light trucks - pickups, sport
utility vehicles (SUV) and vans - increased
by 50 percent in the United States. Since the major
downsizing of passenger cars during 1975-82,
light trucks have had a substantial and growing weight
advantage over cars. By 1992, the number
of fatalities in collisions between cars and light trucks
exceeded the number in car-to-car collisions
In car-light truck collisions, 80 percent of the fatalities
are occupants of the cars. That raises the
question whether the growth in the number and weight of
light trucks is having an adverse impact
on the safety of passenger car occupants and other road
users, possibly exceeding any safety benefits
of the vehicle-weight increases for the occupants of the
trucks.
Vehicle safety is complicated, obviously, and vehicle weight isn't the only variable of merit, obviously. (See
http://energy.lbl.gov/ea/teepa/pdf/aps-ppt-wenzel.pdf )
But all things being equal, in a head on collision between two identical vehicles, save that one weighs a thousand pounds more than the other, the heavier car is going to win.