Callous has an amazing regenerative mutation, allowing him to heal from almost any injury. Unfortunately, his healing factor is constantly “on,” meaning that he’s growing new muscle, fat, and sinew all the time, without his skeleton growing to compensate. He has to cut pieces of himself off from time to time, or he would go blind from skin growing over his eyes, be unable to move from being too heavy, or otherwise become an incapacitated ball of meat. He’s effectively a human tumor.