One word - Leverage - with leverage, the shorter guy wins. The fact that AD99 also had an electric first step and was often blowing through two blockers before either guy had even left their stance certainly helped. The fact that AD99 was strong enough to shove blockers into the paths of opposing QBs and RBs helped more. The fact that AD99 was so fit that he could bring relentless pressure all day long was the icing on the cake.
AD was so good that the Horns hardly attempted to truly elevate the DL around him the prime of his career. When the coaching brass informed him before the 2022 draft that they were ridding the team of most of the big salaried players in favor of rookies | younger pros to establish better cap stability, the Aaron offered the blueprint to success: "Just make sure that they care!"
Instead of the folly of trying to replace AD with one player, the Horns started going after multiple players with high passion, high gridiron IQs, and relentless practice/video study habits. These young defensive studs we now have are pretty much Aaron's spiritual DNA kids.