The main problem is width, not length. Door beams, electric window motors, big wing mirrors, speakers etc have all added to the width, together with fatter people needing bigger seats and central consoles for cup holder, ICE, phone storage etc.
Comparing (say) a Ford Anglia with a current Fiesta (sort of a descendant):
Anglia - kerb weight 737kg, length 390cm, width 143cm
Fiesta - kerb weight ~1220kg, length 407cm, width 174cm
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