I think it is the height, bone structure and thickness of my lower body. I was 225 by age 18. Most guys we compare body weight to are 5'7-5'10. Most of us are not over 6'0, and my weight has always been deceiving.
But anyways, I stay at about 9-11% b.f. year yound and try to get into comp. shape (235lbs) once or twice a year.