mid-15c., "a heap," earlier "ship's cargo" (mid-14c.), from a Scandinavian source akin to Old Norse bulki "a heap; ship's cargo," thus "goods loaded loose" (perhaps literally "rolled-up load"), from Proto-Germanic *bul-, from PIE root *bhel- (2) "to blow, inflate, swell" (see bole).
Meaning extended by confusion with obsolete bouk "belly" (from Old English buc "body, belly," from Proto-Germanic *bukaz; see bucket), which led to sense of "size," first attested mid-15c.
"swell, become more massive," 1550s (usually with up), from bulk (n.). Related: Bulked; bulking.