late 14c., proporcioun, "due relation of one part to another," also "size, extent; comparative relation of one thing to another in size, degree, number, etc.," from Old French proporcion "measure, proportion" (13c.) and directly from Latin proportionem (nominative proportio) "comparative relation, analogy," from phrase pro portione "according to the relation" (of parts to each other), from pro "for" (see pro-) + ablative of *partio "division," related to pars "a part, piece, a share, a division" (from PIE root *pere- (2) "to grant, allot"). Also from late 14c. as "relation of body parts," hence "form, shape." Phrase out of proportion attested by 1670s.
My fortunes [are] as ill proportioned as your legs. [John Marston, "Antonio and Mellida," 1602]
"to adjust or regulate the proportions of; to form according to suitable or harmonious proportions," late 14c., proporciounen, from proportion (n.) and in part from Old French proporcioner and directly from Medieval Latin proportionare. Related: Proportioned; proportioning.