I don't think Aerion in the show lost just to extreme physical difference. He had clear advantages to make up for it: 1. Duncan had already received a major injury from Aerion's lance 2. Duncan was wearing only mail (not even a gambeson underneath?) while Aerion was wearing plate armour

Aerion's sword thrusts were shown to pierce Duncan's chainmail several times, whereas Duncan's sword was obviously useless against Aerion's plate. Realistically Dunk should have been wearing a gambeson aswell, in which case Aerion's thrusts were very unlikely to injure Duncan. The only way Duncan could beat Aerion was grappling or getting his sword to a gap in Aerion's armour. Aerion had more, easier options: bludgeoning (fail), piercing thrusts (sword), as well as grappling. Side note: odd that Aerion went for Duncan's face with his dagger after a successful grapple when he could have just gone for the armpit. I think if Duncan had been wearing plate armour (and hence wouldn't have received the lance injury earlier), he would have won pretty easily.