The Japan Mini Vehicles Association announced recently that the number of subcompact cars on Japanese roads now exceeds 20 million. The K-class of tiny vehicles now accounts for 27.6 percent of the total cars owned in Japan and is the highest in 28 years. The association thinks the steep increase - five million in a decade - is due to changes in subcompact standards in 1998 (that allowed slightly larger dimensions and engines) and owners holding on to their cars for longer periods.