It's called your "set point" weight range, and here are some key facts:

• Keep in mind it's a range – meaning that your weight will fluctuate within that range.  It's completely unrealistic to expect your weight to stay the exact same all the time.  On a daily basis your weight can fluctuate +/- 5 pounds depending on fluid shifts, digestion, etc. 
• It is primarily determined by your genetics.  
• The range will change over time and gets higher as we age.  
• A higher set point later in life is actually protective of health, and those in the "overweight" category live longer than those in "normal" weight category, suggesting that weight gain is beneficial to our health.
• The way we carry our weight changes over time too.  This is especially true for women as they go through puberty, childbearing years and then menopause.  The body's shape changes in response to lifestage.
• Chronic dieting increases the set point weight range.  The body is defending against the perceived threat of starvation.
• Consistently overeating and prolonged weight gain will make the set point range increase.
• Your body's set point range is unique to you.  It has nothing to do with a BMI chart or any other arbitrary way of guessing what a person should weigh.
• It is the range in which your body will function optimally.