I think the whole safe thing is more a factor of size than weight.
Neither.  It's about engineering and construction.  See the scores for my 2006 Civic?

Actually, I think the IIHS ratings are based on vehicle size/class, so your Civic's "good" performance is relative to other small cars.  An interesting tidbit from the IIHS website:

Q: Why doesn't the Institute crash test the largest SUVs?

A: The forces in the Institute's 40 mph frontal offset test into a deformable barrier are similar to those in a real-world crash between two vehicles of the same weight, each going just less than 40 mph. So a test of a very large, heavy SUV would be equivalent to a real-world collision between two such vehicles -- an unlikely crash scenario because the heaviest SUVs represent such a small segment of the passenger vehicle market. Unless such SUVs were designed with very stiff front ends (a design that could exacerbate incompatibility problems in crashes with smaller, lighter passenger vehicles), their own mass would cause extensive deformation in the offset test. A similar degree of deformation would be unlikely to occur in a real-world frontal offset crash unless the other vehicle were equally large and heavy (or unless the SUV hit a bridge abutment, for example). For these reasons, the Institute doesn't plan to subject the heaviest SUVs to frontal offset crash tests into a deformable barrier.

I think the IIHS is assuming that because most other vehicles are smaller that large SUVs, the other vehicle would probably bear the brunt of the momentum change in a collision.  Too bad for the people in the smaller vehicle.

Does anyone know whether insurance companies take into account how much damage you're likely to do to the other vehicle (or people inside) when setting insurance rates?

Michael

Civic curb weight 2,593 lbs
Chevy Suburban 7200 lbs

In a head on collision, your civic will look like a crumpled up tin can.  The surburban may have some dented sheet metal

I will be floating in a sea of airbags.
Yeah but that doesn't mean the Suburban is safe.  Rather it is dangerous for the other person.  
yea, but in a head on collision between a suburban and a civic, would you rather be in the suburban or the civic?
But what if you had to avoid an 18 wheel truck? Which would yuo rather try to maneuver out of the way. Not all accidents are car to car and you may have more accidents in an SUV than you would in a less dangerous vehicle.
I'd far rather be in my civic in a tight spot requiring maneuvering.
http://www.malcolmgladwell.com/2004/2004_01_12_a_suv.html

Most of us think that S.U.V.s are much safer than sports cars.   If you asked the young parents of America whether they would rather strap their infant child in the back seat of the TrailBlazer or the passenger seat of the Boxster, they would choose the TrailBlazer.   We feel that way because in the TrailBlazer our chances of surviving a collision with a hypothetical tractor-trailer in the other lane are greater than they are in the Porsche.   What we forget, though, is that in the TrailBlazer you're also much more likely to hit the tractor-trailer because you can't get out of the way in time.   In the parlance of the automobile world, the TrailBlazer is better at "passive safety.  " The Boxster is better when it comes to "active safety," which is every bit as important.  

Consider the set of safety statistics compiled by Tom Wenzel, a scientist at Lawrence Berkeley National Laboratory, in California, and Marc Ross, a physicist at the University of Michigan.   The numbers are expressed in fatalities per million cars, both for drivers of particular models and for the drivers of the cars they hit.   (For example, in the first case, for every million Toyota Avalons on the road, forty Avalon drivers die in car accidents every year, and twenty people die in accidents involving Toyota Avalons.  ) The numbers below have been rounded:

Make/Model Type Driver
Deaths Other
Deaths Total
Toyota Avalon
 large 40 20 60
Chrysler Town & Country
 minivan 31 36 67
Toyota Camry
 mid-size 41 29 70
Volkswagen Jetta
 subcompact 47 23 70
Ford Windstar
 minivan 37 35 72
Nissan Maxima
 mid-size 53 26 79
Honda Accord
 mid-size 54 27 82
Chevrolet Venture
 minivan 51
 34
 85

Buick Century
 mid-size 70 23 93
Subaru Legacy/Outback
 compact
 74 24 98
Mazda 626
 compact 70 29 99
Chevrolet Malibu
 mid-size 71 34 105
Chevrolet Suburban
 S.U.V. 46 59 105
Jeep Grand Cherokee
 S.U.V. 61 44 106
Honda Civic
 subcompact 84 25 109
Toyota Corolla
 subcompact 81 29 110
Ford Expedition
 S.U.V. 55 57 112
GMC Jimmy
 S.U.V. 76 39 114
Ford Taurus
 mid-size 78 39 117
Nissan Altima
 compact 72 49 121
Mercury Marquis
 large 80 43 123
Nissan Sentra
 subcompact 95 34 129
Toyota 4Runner
 S.U.V. 94 43 137
Chevrolet Tahoe
 S.U.V. 68 74 141
Dodge Stratus
 mid-size 103 40 143
Lincoln Town Car
 large 100 47 147
Ford Explorer
 S.U.V. 88 60 148
Pontiac Grand Am
 compact 118 39 157
Toyota Tacoma
 pickup 111 59 171
Chevrolet Cavalier
 subcompact 146 41 186
Dodge Neon
 subcompact 161 39 199
Pontiac Sunfire
 subcompact 158 44 202
Ford F-Series
 pickup 110 128 238

Are the best performers the biggest and heaviest vehicles on the road? Not at all.   Among the safest cars are the midsize imports, like the Toyota Camry and the Honda Accord.   Or consider the extraordinary performance of some subcompacts, like the Volkswagen Jetta.   Drivers of the tiny Jetta die at a rate of just forty-seven per million, which is in the same range as drivers of the five-thousand-pound Chevrolet Suburban and almost half that of popular S.U.V. models like the Ford Explorer or the GMC Jimmy.   In a head-on crash, an Explorer or a Suburban would crush a Jetta or a Camry.   But, clearly, the drivers of Camrys and Jettas are finding a way to avoid head-on crashes with Explorers and Suburbans.   The benefits of being nimble--of being in an automobile that's capable of staying out of trouble--are in many cases greater than the benefits of being big.  

Tire selection and maintenance is an important augment for active safety.  There are some very good handling & braking tires out there.

All in all, my 1982 M-B 240D was about the safest (active & passive) car made when new.  Now, it is still among the better ones, despite no airbag.

I have been in a number of salvage yards, and seen dozens of crushed W123 M-Bs.  IMHO, the driver could have survived everyone with  minimal injuries if belted in.

BTW, I get 31 mpg in the city with my manual transmission.  This is the car IO chose to see me through Peak Oil.

I think the technology of safety belts has improved significantly since then (basically a bit of explosive that stops you hitting the dashboard or the wheel by locking the belt-- you may bruise your shoulder and break a few ribs, but you survive).  Don't have any cites, but this is my impression.

In addition, cars then didn't have rear shoulder belts, so an additional risk is present if you have passengers.

Also anti-lock (ABS) brakes were invented later on.  They make a big difference to collision avoidance, I believe.

Agree they were nice cars although if I can recall correctly they needed a lot of TLC (frequent oil changes etc.).