How is it determined?

I see some people are considered "lightweights" how is that determined?

Lower Bmi basically.  As far as I know there's no official categorization but if you are a lower Bmi 38-42 you are a lightweight compared to people that have a Bmi of 50,60 etc.  Generally your chances of getting to a normal Bmi or even underweight are greater than those with more to lose etc 

People sometimes consider themselves light weights if they start at say 250 instead of close to 400. The more you have to lose the faster you will lose if you follow the program