Mondami - you're right, you probably have more muscle mass which makes you look miles thinner, while also making you weigh more.
Also, it is so much about body shape. When you are bigger on top, or carry weight on your face you always look like you weigh more, than someone who carries their weight on their bottom half.
Am sure the dresses were all picked at the end to be the size they had become no matter what it was.
Am also sure most of them will out the weight back on as a) they lost a lot very quickly and b) they didn't seem super-commited. One of them was saying about how she cheated and ate chocolate. Not the end of the world obviously but was surprised that she didn't stick to the diet when it was only 2 weeks, and she was going to be on TV, but then if dieting was that simple, everyone would be succeeding at them.
Good luck to them all though, they seemed pretty chuffed at the end, and all seemed realyl nice folks.