I guess one would have to distinguish between the old Buddha statues, produced from around 350 to 800 that were carved directly out of stone cliffs, and the free standing ones of later date. Buddhism reached East Asia via the silk roads where it received visual influence from Greece. Along the eastern ends, as well as the western edges, developed many temple cave complexes, such as Dunhuang and Mogao. There the early monastic communities received patronage from the court, traveling merchants and urban communities to the east. This led to some ambitious cave and statue projects. The size of the Buddha would signify the effort and resources that went into construction. This in turn would reflect well upon the patron. Later urban Buddha statues like the ones in Burma or Japan pick up on this ancient tradition of going big.