I think for some people you just average out at a higher weight....for example, me and my sister were raised on the same diet growing up. I hated exercise and always found ways to get out of PE, while she did rugby, football, strength training, everything that was offered to her. When we hit our teen years I started smoking, binge drinking and eating loaaaads whenever I had munchies. My sister was going to social clubs, using her school's gym and joined a hiking group.

When it came time for us to both move out, I was 'skinny' and she was 'fat', easily twice the weight of me. Now people call me the healthy one, because all they see is the weight. But even at her healthiest, she never became 'skinny,' and she never lost weight, while I never put it on despite all my bad choices. I've just always thought that people get build different!