It makes no distinction between types of mass: ie. fat vs. muscle vs. bones vs. organs etc. I could gain muscle mass without losing fat and my BMI would still go up. This index is meaningless.
A good fat indicator is [link|http://www.healthchecksystems.com/bodyfat.htm|percentage body fat]. Work on reducing your percentage body fat to a realistic level and don't worry about how "massive" you are.