Re: Well, as a fatty...
There are certain recognised body types, you sound like an endomorph or mesomorph (http://www.muscleandstrength.com/articles/body-types-ectomorph-mesomorph-endomorph.html)
The whole BMI thing is crap. at 6' 6" and about 16.5 stone, I'm overweight. Not badly, but still overweight. I work as a web developer so I'm generally not that active during the day, but I also train in powerlifting, go climbing regularly and have a 30" waist, so not a lot of it is excess fat. Still, doctors recommend that I lose weight.
It should be based on a combination of height, body weight, body fat percentage and body type, as well as various blood markers.