It is a common myth in Pakistan that older cars are heavier. It is not true especially in case of Japanese cars. It is probably true for older american cars but not Japanese cars. You can compare weights of different generations of same car like corolla by going online. You'll find out for yourself.
Stricter safety requirements around world has caused Japanese car chassis to become stronger over the years. Outer body panels might have become lighter but chassis has become stronger and heavier
