Yes Alister Vacbear,
the Viva was a subcompact by the US industry ratings, same as the Chevette made here, a T body car. It's precdecessor the Vega as well, on the former H body, along with it's luxury variants the Monza, Pontiac Sunbird, Olsmobile Starfire, and Buick Skyhawk. Our full size cars began shrinking in 1977. GM cars lost several inches in length, and about 800 lbs. on average while maintaining almost the same interior and boot volumes. Wheelbases shrank from 121 inches to 116 for GM B bodies, and from about 126 to 120 on C bodies except the Cadillac Fleetwood 75. In 1978 mid size cars got the chop by 600 lbs., and wheelbases from 116 to 108 inches (like many 70's compacts), ecept the ElCamino, which actually grew from 112 to 116 inches. Thinner doors and stationary rear door windows allowed for added space. By then, no hardtop coupe had an opening rear or sail panel window.
Once the industry began to shift to front wheel drive, the size categories changed. With a flatter floor, many compacts became mid size cars, and sub compacts compacts. In about 2002, the SAE got the parameters changed again. The Ford Taurus for ex. which had been a mid size, became a full size, more in line with European standards. Even though it's floor pan platform had not changed yet. A slightly higher roofline enabled enough exrtra interior volume for it to qualify.