The two gas crisis and the tightening of emission standarts took a toll in the traditional American automotive industry. You'd had monstruous cars with underpowered engines and a increasing reputation of being unreliable, while the foreign automakers had smaller, more efficient and reliable vehicles. This way, by the late 1970's and 1980's, Japanese and European automakers were already positioning and strengthening withing the American market, a tendecy which would continue throughout the decade.
Now, judging by footage and stuff from the 1980's, by the mid and end of the decade compact and subcompacts cars were already heavy on the streets. You can see a lot of Datsun, Escorts, Tempos, Chevettes, Horizon/Omnis, Golfs, Excels, Colts, Corollas, Tercels, Civics or Accords, Mazdas, or even compact pickups, like Nissan/Datsuns, Toyotas or Chevy S10. Some American automakers significally resized their product lines as well, like Chrysler with their K car derived cars.
I think there are geographical area of the country also had some influence. The West Coast seemed to have a bigger presence of foreign cars, that is, compacts and subcompacts, I think due in part to the locations of the maritime ports where foreign automakers would unload their vehicles and to more severe emission laws, while the Midwest, home of the American auto industry, was still heavy on American cars.
This footage from 1988, featuring a Californian highway, show how large compact vehicles were already in number by then, while in this one, shot in New York City in the early 1980's, compact vehicles are harly seen.