Leptin and fat mass?
The obese gene (OB gene) secretes a protein that functions in the signaling pathway of adipose tissue and adipocytes (fat cells) produce the hormone leptin (reference). 


  When the amount of fat in the body increases, the amount of leptin
  released into the blood also increases and signals to the brain that
  the body has enough to eat (reference). Inversely, studies in rodents suggest 
  that a reduction in leptin level during weight loss signals to the brain 
  to increase feeding and decrease energy expenditure (reference).


Other molecules have a role to play in the ratio of BMI to leptin like the ghrelin hormone (reference). The genetic and environmental factors other than overall adiposity may also regulate leptin concentrations (reference) which is why individuals with the same weight may have different leptin concentrations. 

A study has reported finding a strong exponential relationship for leptin levels with body mass index (BMI) (reference). Hope that answers both your questions.