A current compact car is wider than a midsize car from a few decades ago. The growth was gradual, driven by engineering requirements rather than styling ambition.
Side impact protection needs thickness
A door has to absorb energy in a crash while keeping the occupant's space intact, and the only way to do that is with structure occupying real depth.
Intrusion beams, reinforced pillars and energy-absorbing foam all sit between the outer skin and the occupant, and each addition pushes the exterior outward.
Side curtain and thorax airbags require packaging space in the pillar and seat, adding to the same dimension in the same place.
Shoulder room became a comparison metric
Interior width at shoulder height is quoted in specifications and compared directly between competitors, which makes it a target rather than an outcome.
Three-across seating in the rear is judged on that measurement, and a car that seats three uncomfortably is marked down against one that does it well.
Since interior width can only come from exterior width minus structure, and structure kept growing, exteriors had to grow faster than interiors did.
Tires and tracks widened alongside
Wider tires improve grip and braking, and heavier vehicles need more contact patch to stop in comparable distances.
A wider track improves stability and gives suspension geometry more room to work, both of which help vehicles meet handling and rollover expectations.
Fenders must cover the tires through full suspension travel and steering lock, so track width translates almost directly into body width.
Infrastructure did not grow with them
Parking space dimensions, garage widths and many lane markings in American cities date from an era of narrower vehicles and have not been revised.
The practical consequence is that mirrors define the usable width of a parking space, and folding mirrors moved from a luxury feature to a common one.
Older residential garages and urban parking structures are frequently unable to accommodate current full-size vehicles at all, which shapes what people can buy.
Where the growth is meeting resistance
Efficiency regulations penalize frontal area indirectly through aerodynamic drag, which gives manufacturers a reason to stop widening.
Some manufacturers now hold width constant across a model generation and find interior gains through thinner seats and repackaged door structures.
Electric platforms help, since the absence of an engine and transmission tunnel frees interior space without any change to the exterior envelope.