From actual death statistics
http://www-nrd.nhtsa.dot.gov/Pubs/808570.PDF
"Abstract
Fatality rates per million exposure years are computed by make, model and model year, based on the crash experience of model year 1985-93 passenger cars and light trucks (pickups, vans and sport utility vehicles) in the United States during calendar years 1989-93. Regression analyses calibrate the relationship between curb weight and the fatality rate, adjusting for the effects of driver age, sex and other confounding factors. The analyses estimate the change in fatalities (including occupants of the "case" vehicle, occupants of other vehicles in the crash, and pedestrians/bicyclists) per 100 pound weight reduction in cars or in light trucks. A 100-pound reduction in the average weight of passenger cars, with accompanying reductions (based on historical patterns) in other size parameters such as track width, and in the absence of any compensatory improvements in safety technology, is associated with an estimated increase of 302 fatalities per year (± 3-sigma confidence bounds range from an increase of 170 to an increase of 434). However, a 100-pound reduction in the average weight of light trucks is associated with an estimated decrease of 40 fatalities (± 3-sigma confidence bounds range from a decrease of 130 to an increase of 50). In car-light truck collisions, 80 percent of the fatalities are occupants of the cars. When light trucks are reduced in weight and size, they become less hazardous to occupants of passenger cars as well as pedestrians, bicyclists and motorcyclists. Conversely, growth in the weight and size of light trucks could increase hazards to those groups."
An updated study with a breakdown of statistics based on type of accident and compensating for safety features such as airbags etc.
http://www.nhtsa.gov/cars/rules/regr...pdf/809662.pdf
"Why heavier vehicles have usually had lower fatality rates
One safety factor, momentum conservation, is a direct consequence of a vehicle’s mass. Other parameters, such as a vehicle’s length and width are naturally and historically (i.e., during 1968-99), but not inevitably proportional to its mass. Most of those parameters favor the heavier vehicle, making it physically, intrinsically safer than the light vehicle.
Some human factors of drivers are historically, but not intrinsically confounded with vehicle mass. For example, young drivers historically have driven smaller cars4, but at least in theory, they might at some future time prefer large cars. These factors could give heavy vehicles lower fatality rates, but don’t make them intrinsically safer. The analysis should, as much as possible, remove these factors and compare the fatality rates of heavy and light vehicles on a level playing field, leaving only the physical factors that make heavy vehicles safer. Finally, there are in-between factors where it is not so clear if the relationship with mass is intrinsic or coincidental.
Momentum conservation: When a heavy and a light vehicle collide, the heavy vehicle keeps moving forward; its occupants experience a small velocity change. The light vehicle gets pushed backward; its occupants experience a higher velocity change. These are consequences of the laws of physics; nothing can be done to equalize the velocity changes. For example, in a head-on collision, a 1 percent weight advantage corresponds to more than a 5 percent reduction in the driver's fatality risk, relative to the driver of the other vehicle.5
What benefits an individual – being in the heavier of the two vehicles – however, does not necessarily benefit society as a whole. Based on momentum considerations alone, the risk reduction in Vehicle 1 as it becomes heavier is cancelled by a risk increase in Vehicle 2. If momentum conservation were the only factor making heavier vehicles safer (it isn’t), overall fatalities in multivehicle crashes would neither increase nor decrease if the entire vehicle fleet were reduced in mass."
other statistics from this report
net increase of deaths resulting from a 100 pound reduction in weight (all types of accidents)
Trucks > 3870 lbs 71
Trucks < 3870 lbs 234
Cars > 2950 lbs 216
Cars < 2950 lbs 597
heavier is safer QED