No matter how cute, peppy or easy to park minicars cars may be, the simple and sobering fact remains: size matters in vehicle safety.

The smallest cars on the road — often called minicars, microcars, or city cars — just don’t measure up to larger vehicles in the critical job of keeping their passengers alive, according to tests conducted by the Insurance Institute of Highway Safety (IIHS). Though advances in vehicle safety caused death rates to drop by more than a third in the past three years, smaller cars still have to contend with the basic laws of physics. 

Data analyzed by the National Highway Traffic Safety Administration (NHTSA) show that occupants of cars that weigh less than 3,000 pounds have higher fatality rates than occupants of larger, heavier vehicles. Four of the top five cars with the highest rates of driver deaths are minicars, according to a study of fatal crashes from 2009-2012.

RelatedWhy You Should Let Your Teen Drive the Newer Car

Why size matters

Larger cars, SUVs and minivans are designed with substantial crumple zones in the front and rear that deform on impact to absorb energy. They were invented in the 1950s by an engineer for Mercedes Benz, who designed a rigid, high-strength passenger compartment protected by softer sections that collapse in a crash. On most modern cars, the frame and body are built as one unit, with the frame, inner fenders and body panels working together to provide a safe, predictable deformation.

Most minicars are designed the same way as larger vehicles, but the crumple zones are considerably smaller. Without the additional space, there’s less ability to absorb impact without transferring that violent energy into the passenger compartment.

Small cars compensate for the lack of large crumple zones with a reinforced shell around occupants. The combination of a rigid passenger compartment and ample airbags has helped improve the crash-test performance of many small cars.

RelatedIs All-Wheel Drive Safest?

But in 2012, IIHS added an additional test that mimics what happens when the front corner of a car collides with another vehicle or an immovable object, like a tree or utility pole. About 25 percent of serious injuries and fatalities in head-on crashes occur in these types of collisions. Ten of 11 minicars tested failed this test, with poor or marginal scores. Only one, the Chevrolet Spark, received an acceptable rating and scored well enough overall to win an IIHS top safety pick in its class. 

But here’s the catch, according to IIHS: Crash test ratings can’t be compared across vehicle weight classes, so a minicar with an acceptable rating will not protect as well as a larger car with an acceptable rating.

If you own a minicar or are thinking of buying one, there’s this ray of hope: smaller cars are now as safe as big cars were 20 years ago, according to NHTSA. 

For some drivers, the safety concerns may be secondary to the excellent gas mileage and low cost of minicars. And minicars are bound to get safer: The latest ones — and even the smallest microcars — are being re-engineered for better performance on stringent new tests here and in Europe.

RelatedRoad Trip Checklist for Your Car

Mike Saunders is a former Boston Globe writer who spends entirely too much time in his garage.