The bulk of something is the greater part of it. Most, by a clear margin.
It takes of and it is always singular, so the bulk of the work was done by two people. There is a separate sense meaning great size, and a man of considerable bulk is a large one. Trade uses it as an adjective, so buying in bulk means buying large quantities cheaply, and a bulk order is a big one. The verb bulk up means to gain muscle deliberately.
Bulk was cargo. Old Norse búlki meant the freight in a ship's hold, from a word for a heap or a pile, so buying in bulk is buying by the shipload.
Proto-Indo-European
*bʰelǵ- — beam, pile, prop
Proto-Germanic
*bulkô — beam, pile, heap
Old Norse
búlki — the freight or the cargo of a ship
Middle English
bulk
English, today
bulk
Built from
From Middle English bulk, bolke (“a heap, cargo, hold; heap; bulge”), borrowed from Old Norse búlki (“the freight or the cargo of a ship”), from Proto-Germanic bulkô (“beam, pile, heap”), from Proto-Indo-European bʰelǵ- (“beam, pile, prop”). Compare Icelandic búlkast (“to be bulky”), Swedish - dialectal bulk (“a bunch”), Danish bulk (“bump, knob”). Conflated with Middle English bouk (“belly, trunk”).
Etymology from Wiktionary, CC BY-SA