What happens when a Shapechanged Dragon enters an Antimagic Field inside a narrow space? [duplicate]

A rather simple question: if an Ancient Dragon uses it’s Shapechange ability to transform into a medium-sized humanoid, and then has that Shapechange disabled/dispelled inside a narrow tunnel / small underground room, will they get crushed to death by their own size? I have plans on True Poly-ing my spellcaster PC into a Dragon, and i’m worried that a random dispel would instakill me when i go dungeon-delving in the Abyss or somewhere.