"BMI is just a calculation based on height and weight, and doesn't tell you anything about body composition. Medical professionals just use statistical correlations to determine when your risk of having weight-related health problems go up--ie, having a BMI above 30 makes you statistically more likely to have certain medical problems. But a short bodybuilder with huge muscles could have a BMI of 30 with very little body fat, and a tall, fairly thin person with poor muscle development could have a low BMI with very high body fat percentage. So, bust size could increase your BMI and make it look like your health risks are higher than they are, and you'd need a body composition analysis (DEXA-bone density-would be best, skinfold calipers second best in this case--underwater weighing would probably be misleading for people with a large bust) to tell you how much fat you actually have to lose.