The 1950's and 60's so called, Yank Tank fell victim in 1973 to the worldwide oil crisis, which resulted in the enaction of CAFE fuel economy standards in 1975.
The CAFE standards required all new automobiles to have an average mpg of 27.5 mpg (11.7 km/litre) by 1985. As of 2013 the 27.5 mpg standard remained unchanged for passenger cars, with light trucks and SUV's required to meet a standard of 23.5 mpg (10 km/litre.)
CAFE: Corporate Average Fuel Economy.
As for British cars being small, even the famous Mini has grown so big that it just about squeezes into a marked parking bay, these days. Even if it has retained it's "Mini" badge.