One of them was lying about their height. At the start it said they were both 5'5", but she is clearly taller by an inch or so. My experience of men says he is actually 5'4" but pretending otherwise.
Also, did you see how thin her sister was? It was only a quick shot of the family eating together, but she was really thin too.
Smigget - the point of the programme is that neither diet is any healthier than the other. For the obese person, it's good to watch someone else struggling to eat all they do, and for the skinny one it's an opportunity to broaden their range of foodstuffs.
Vice versa, the skinny one gets to see how paultry their diet is, and the obese one realises they can survive on less food. It's a matter of having a different perspective.
The 12 week eating plan they're on afterwards is healthy.