The forms you're looking at are actually the ancestors of「矩」.「巨」was formed by cutting off the person.
The analysis starts from this form
which depicts a person「大」holding a tool「工」.
Carpenter's square is much too specific, and I would think that it is an over-interpretation.
Exaggerate the person's hand to emphasise that the tool was held:
Swap out「大」with the similar-meaning「夫」(man), and permanently detach part of the hand onto the tool:
The left hand side of the above character should now heavily resemble「巨」, which is created from omitting「大」or「夫」.
「夫」was later semantically corrupted into「矢」(arrow > length, measurement), forming「矩」.