Quit mythologizing the 80's as some physical, defensive oriented era. Just because they occasionally took each other's heads off like barbarians doesn't mean physical play was the rule. Rewatch games from the period, and you'll see some of the worst defense imaginable. Hardly any double teams, chaotic team defense, God awful transition defense. Why do you think there were so many 28ppg+ scorers on 50% plus shooting during that time?
McHale "beasted" on those same bigs, and Duncan's is a far superior version to McHale. But yeah, Duncan would struggle against the "amazing" bigs of the 80's where McHale didn't
Furthermore, I've already debunked your claim that Duncan "made a living" out of matching up against smaller players. He was always guarded by the opposing team's best interior post-defender, often facing double and triple teams. You should remember when they tried to put a prime Shaq on him and he torched him.