[icosa]

BMI uses height and weight to estimate body fat % because those were the easiest statistics Quetelet had access to. There's speculation that BMI is effectively a proxy for waist size, given assumptions about body shape (how high the person's waist is) and density (fat to muscle and bone ratio).

Waist-circumference-to-height ratio is a proven predictor of cardio health, and is only slightly harder to measure than BMI. Waist-to-hip-ratio is another strong predictor for health outcomes, and can flag people with a normal BMI that have an elevated mortality risk.