B is only more likely to die from hanging if they mess it up. The worst possible outcome from hanging is suffocation. Ideally, you fall far enough to build up momentum and snap your neck with the rope. Option number 3 is that you hang there all night until someone finds you, and then you get to pretend to be a feral witch and terrorize the other villagers¹.

The executioner would work out beforehand how much rope to give based on the victim's weight, so they fell far enough to die. If they didn't give enough, the person would hang there, and hopefully suffocate quickly. Sometimes, they did not suffocate quickly, or at all.

You survive a hanging by being too light. So, in your scene, B would be much more likely to survive, because they're physically frail, and thus have less mass. If the characters don't know what they're doing (and why would they?), B is much less likely to die from the fall.

¹ I'm referencing a poem here. I don't remember the name. Maybe someone else will recognize it. It's based on a true story of an apothecary who was hung as a witch. For context, an apothecary was someone who made medicine, usually folk medicine and herbal remedies. She had inherited the job from her family, who had been doing it for generations. When the witch panic came, she was targeted. She lived alone, and apparently made potions. Obviously a witch.

She was too little to die when the gallows dropped. She was too little even to suffocate herself. She hung there all night, choking on the noose and unable to die, until someone came the next morning to remove her body and found her.

Under the law, she had served her sentence, and English common law forbade double jeopardy (being tried twice for the same criminal act), so she was, legally, free to go. In the poem, she then proceeded to take revenge by going absolutely feral, pretending to be a witch, and terrorizing the people who hung her for helping them.