Assuming that people are approximately equally dense
Not quite true, though. Bone is twice as dense as fat, and muscle about 10% more dense than fat.
BMI favours people with average to slim shapes. If you are broad across the shoulders/chest/hips (in basic skeletal structure), and then well muscled on top, you'll have a high BMI regardless. Not many prop-forwards have a BMI less than 28, but I wouldn't call them unfit.