mid-15c., "a heap; the volume or bulk of something," earlier "ship's cargo" (mid-14c.), from a Scandinavian source akin to Old Norse bulki "a heap; ship's cargo," from Proto-Germanic *bul-, from PIE root *bhel- (2) "to blow, swell."
Meaning extended by early confusion with obsolete bouk "belly" (from Old English buc "body, belly," from Proto-Germanic *bukaz; see bucket), which led to sense of "size, volume, magnitude of material substance," attested from mid-15c. In bulk 1670s, "loaded loose." Meaning "the greater part" (of anything) is by 1711.