LOL, he wasn't wearing double armor :) When you hit level 20 you get "Appearance Slots". Hit "I" for your inventory, there should be a second tab which says "Appearance". You can put stuff in these slots and their appearance overwrites the look of what you're wearing.
As for why he was so much more buff than you, yes, it can be armor, but he might have also had buffs on from a group.