What does Boxing Day have to do with boxing?
late 14c., "due relation of one part to another," also "size, extent; compartative relation in size, degree, number, etc.," from Old French proporcion "measure, proportion" (13c.), 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 (see part (n.)). 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," late 14c., from proportion (n.) and in part from Middle French proporcioner and directly from Medieval Latin proportionare. Related: Proportioned; proportioning.