actually, in all seriousness
my first impression was that it's simply an alpha-male, evolutionary thing. males fight all the time in all species, competing for females.
you're asking a few questions there, and i'm not sure how to address each of them. as far as why is the western society seemingly always so backwards, i'm not sure that western society actually is backwards. i'm sure all societies have different complexes, based on their myth structures and religions.
i'm not sure when the size thing started. it definitely exists now, thats for sure. i'm not sure if it's different than the female obsession with their bodies which is a much more harmful obsession, to be sure. so to address the question i'd have to wonder whether it's just a manifestation of the marketing age or if it has older origins. so i'm not so ready to blame it on christianity. and i'd be hardpressed trying to find a source there, anyway.