% of starting weight would have to determine the winner.
For example:
If someone starts at 200 lbs and loses 12 lbs, that's 6% of their starting body weight. If someone starts at 120 lbs and loses 10 lbs, that's 8.3% of their body weight and much harder to do. The second person would win even though technically, they've lost less lbs than the other person.
The biggest loser does it this way so that a woman who weighs 200 lbs can still compete on the same playing field as a guy who weighs 400 lbs. The actual lbs lost is less, but as a % of starting weight, it can be the same (or better).
To calculate: % = lbs loss/starting weight x 100.