1973 bumpers: significantly lighter than '74+?

It's not the bumper itself. '73 & '74 with standard bumper brackets have a reinforcement plate sandwiched between the bumper & standard brackets that can be removed. The '75-'76 "shock absorber" bracket type bumpers have a very heavy reinforcement that can't simply be removed because the "shock absorber" bumper brackets bolt to to it, not through it. Only the '76 Feather Duster & Dart Lite bumper assemblies are closer to the older style "channel" brackets because their reinforcement is made from aluminum.