I think one reason those people on Biggest Loser (haven't seen it) can lose weight so quickly is because the bigger you are, generally the faster you can lose weight. Those last 5/10/15 lbs really are the hardest, the thinner you get, the more your body seems to hold on to the fat. But if it knows it has 200 lbs excess fat, hey! What is 10 pounds?
Of course, some obese people have difficulty losing ANY weight. Hormonal problems.