What alternate dimension are you from? My older brother had a 60's Ford Fairlane when I was growing up. My first car was a 67 mustang. I had a friend with a 68 Camaro. I knew someone else that had multiple Thunderbirds from that era.
These were not small cars. The Mustang and Camaro were the size of a full size sedan today , if not bigger. The cross sections of the doors were very large, they just aren't very space efficient at all. They're only small relative to the other cars of the era, and I wouldn't call their size reasonable in any real way.
Those are the coupes. The sedans, such as the Fairlane and Thunderbird? Those were about as big as a modern full size crew cab pickup.