Volvos are safe for two reasons.  First they do a very nice job of safety engineering and second they tell you that they did a very nice job of safety engineering.  Unfortunately, Porsche is a very small company and has a laundry list of praiseworthy attributes.  The bottom line is Porsche has a lot to brag about but can't afford to do the bragging.

Passive safety is what the vehicle does to protect its occupants after it becomes involved in an accident.  From Porsche's earliest days, safety has been an extremely important issue.  For those old enough to remember, when involved in front end collisions, automobiles used to wreaked havoc on their drivers.  The steering column ran straight back to the steering wheel and would force the steering wheel into the driver.
When Porsche entered Formula 2 racing, it took its 550 spyder and moved the driver's seat from the left to the center of the vehicle.  Instead of reengineering the entire steering mechanism, they added two universal joints to reposition the steering wheel.  It didn't take long to realize that the steering wheel was no longer a serious threat.  This discovery was immediately shared with the entire automotive industry.  A design flaw was eliminated to the benefit of all drivers.

While the previously mentioned small company had its disadvantages, being small meant the engineers knew the drivers of the racing cars.  At most automobile manufacturers there is a trickle down of technology from racing to production.  To quote the late Bob Snodgrass of Brumos Porsche, "At Porsche it's a flood."  The drivers' safety was always paramount with Porsche engineers. 

As early as the mid 60's Porsche was crash testing all its cars, both racing and production.  A crane would lift a Porsche and drop it to simulate a crash.  Today's methods are little more sophisticated.  It quickly became apparent that the more likely scenario of a front end collision would be offset rather than head on.  Porsche's active safety would get at least part of the vehicle out of harm's way.

Look inside the door jamb of any late model Porsche sports car.  At the back of the door jamb near the top, you will see a wedged shape object painted a whitish grey.  There is a rod that runs lengthwise through the upper part of the door.  This is designed to transmit energy from an offset collision from front to rear or rear to front and contribute both to the strength of the safety cell around the passenger compartment, as well as the absorption of energy.  In most convertible designs, the entire load has to be absorbed by the rocker panel beneath the door.

Now, while you're there, look at the side of the seat's backrest.  A thorax airbag is stored in there.  Why in the seat?  By placing the thorax air bag, part of Porsche's occupant side impact protection (POSIP), in the seat, it is always in the proper position to afford optimal protection.
Back to the safety cell, all Porsches utilize a very rigid structure designed to prevent intrusion into the passenger compartment.  The rest of the vehicle is expendable and designed to absorb energy at constant rate.  If the entire vehicle were rigid, the occupants would be subject to all the energy of the sudden stop (front end) or sudden acceleration if hit from the rear.
Porsche utilizes different materials and varies the thickness of the metal (tailored blanks) to facilitate energy absorption.  The end result is that the vehicle is expendable not the occupants.

Porsche's cabriolet designs either have fixed rollover bars (Boxster) or pop-up rollover bars (911).  Look at the angle of the roll bars.  They are designed to permit the vehicle to continue rolling to afford the best chance of landing right side up.  If a roll bar is squared off, it may prevent the vehicle from returning to the preferred shiny side up position.
Like the old saying, "It's not the size of the dog in the fight," A larger vehicle is not necessarily a safer vehicle.  Porsche engineering is always at the leading edge of vehicle safety.  That's where I want my family.