Engines have become smaller and considerably more efficient over the past decades while the vehicles carrying them have grown steadily heavier. The added mass came from everywhere except the engine bay.
Crash protection is structurally heavy
Modern vehicles are designed to deform in controlled ways, absorbing energy through crumple zones while a rigid passenger cell resists intrusion.
That requires reinforced pillars, side impact beams, high-strength steel in the floor and roof, and mounting points strong enough for multiple airbags and restraint systems.
Each generation of testing has raised the standard, and the improvements have generally been achieved with additional structure rather than by removing it.
Comfort expectations added mass everywhere
Buyers now expect a quiet cabin, which is achieved through sound deadening material, acoustic glazing, thicker seals and additional structural stiffness.
Seats have grown more complex with adjustment motors, heating, ventilation and reinforced frames, and each adds weight in a location high in the vehicle.
Air conditioning, larger displays, powered tailgates and multi-zone climate control all bring wiring looms that have themselves become a substantial component.
Vehicles simply became larger
The popular segments shifted towards taller, wider bodies, and a model nameplate carried across generations has typically grown at every renewal.
A larger footprint requires bigger brakes, wider tyres, stronger suspension and more body panel area, so growth compounds through the whole vehicle.
Ride height adds further structure, since a taller body needs additional bracing to control roll and to meet stability requirements.
Electrification moved the mass rather than removing it
Battery packs are dense and large, and a pack sized for long range weighs more than the engine, transmission, fuel tank and exhaust it replaces.
The pack also requires a protective enclosure and cooling system, and the surrounding structure must be reinforced to keep it intact in a collision.
Hybrids carry both systems, which is why they often weigh more than either equivalent alone.
Weight undoes part of the efficiency gain
Accelerating and stopping mass consumes energy regardless of how efficient the powertrain is, and heavier vehicles need more of both.
Tyre and brake wear rise with mass as well, and those produce particulate emissions that are unaffected by how clean the propulsion system is.
Road wear and the severity of collisions with lighter road users both scale with weight too, which is why vehicle mass has become a policy question rather than only an engineering one.