Monsters are far, far more than just HP and damage dealt. There is no 'scaling'. Monsters have no level (any more). The monster's instinct, moves and their attacks (not damage dealt) are the key to their malignancy and unique appeal.
In your Dungeon World, A Dragon is simply a large scaly beast of suitable HP and damage dealt to challenge the increase of HP on a higher level character.
In my Dungeon World, Dragons are NAMED ancient wyrms that lurk in places deep and long forgotten by civilised folk. They forge despotic alliances, hoard treasure, eat maidens, sleep deep, offer duplicitous bargains and seek greater magics.
When roused into fighting they isolate their chosen prey, pin with their talons, scorch with fiery breath, buffet with leathery wings and smash with quivering tails.
When threatened they cast ancient magics of protection, terrorise the countryside, seek the safety of the skies and ponder fearsome and intelligent retribution.
They have supple, wondrous bodies, covered with armoured scales that offer 4 armour, and only weapons quenched in the heart's blood of another dragon, or made with dragon tooth, bone or scale will harm them at all.
Though I wonder (as a stake) if legend speaks truth and each beast has an 'achilles heel' to breach their potent defence - to wound their strong, emotional heart.
Oh, and if you care to know they have between 40-60HP, deal 40-60 Damage and have + messy, +3 Piercing, + Reach, + Far, +Long.
I am a fan of the characters, truly I am.