bulkyoudaoicibaDictYouDict[bulk 词源字典]
bulk: [14] Formally, bulk comes from Old Norse búlki, which meant ‘cargo’ or ‘heap’: the original connotation of the English word in this sense was thus of goods loaded loose, in heaps, rather than neatly packed up. That is the source of the phrase in bulk. However, a certain similarity in form and meaning to the English word bouk ‘belly’ (from Old English būc, and ultimately a descendant of West and North Germanic *būkaz) led to the two being confused, so that bulk was used for ‘belly’, or more generally ‘body’.

Modern connotations of ‘great size’ seem to be a blend of these two. The bulk of bulkhead [15] is a different word; it may come from Old Norse bálkr ‘partition’.
[bulk etymology, bulk origin, 英语词源]
bulk (n.)youdaoicibaDictYouDict
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.
bulk (v.)youdaoicibaDictYouDict
"swell, become more massive," 1550s (usually with up), from bulk (n.). Related: Bulked; bulking.