Mass matters in all accidents - doesn't matter which direction they are coming from.
Accidents in which a passenger car is hit by a semi-truck are fatal 0.64% of the time.
In contrast, accidents where a car rear-ends a semi-truck are fatal more than twice as often, 1.6% of the time.
http://www.fmcsa.dot.gov/facts-research/research-technology/analysis/rear-end-crashes.htmWhich says two things: 1) Rear-end accidents are rarely fatal regardless of size differential, and 2) the direction its coming from absolutely does matter.
It isn't the actual force of impact that does the damage, its the rate of acceleration or deceleration. When you get hit from the rear your vehicle moves forward, and that movement absorbs most of the energy. In fact, of the fatal accidents from being hit by a semi-truck, nearly half involved the car being struck hitting another car ahead of them. (Which, incidentally, means the car driver was probably tailgating). It is the impact into that car that does the damage. Controlling for multi-vehicle accidents, it leaves only a 0.34% fatality rate for being hit by a semi.
If the mass differential of a semi-truck - 80,000lbs - vs a car - 4000lbs - is so insignificant, what do you think the impact of mass is on a rear-end collision between a 2000lb car and a 4000lb car?
As far as the risk of you hitting another car, if you are safety conscious enough to want a safe car, you should be safety conscious enough not to speed or tailgate, and if you don't do those things the risk of you rear-ending someone is practically zero.
Stopping distance is directly related to weight but also to the coeffcient of friction (tires with larger surface area and softer compounds and the increased weight of the vehicle can reduce stopping distances).
Ford Focus, Weight 2962lbs / 60-0: 114ft
Ford Expedition, Weight 5547lbs / 60-0: 142ft
Sure, tires and ABS affect braking as well - I'm talking all other things being equal. You can put better tires on a small car than it comes with. Even a cheap little motorcycle, the EX250R, with its two skinny tires and no ABS, can out brake the Expedition (and most full-size trucks and SUVs) at 121ft
bottom line big vs. small then big will win which makes small cars less safe. It is physics and stats show it.
The stats show the smallest compact cars to have more fatalities, but they show mid size cars to have LESS fatalities than large trucks and SUVs. Even though they weigh less. Remember MMMs post on safety? Remember the chart he posted?
Total Occupant fatality rates:
Midsize SUVS - 16.16
Standard pick-ups - 13.87
Full-size SUVs - 12.34
Full-size cars - 12.16
Mid-size cars - 11.49
A mid-size car weighs less than any of the other categories, yet has the least statistical risk of being involved in a fatality.