Small fat is essentially "still mobile". Think BMI 30-39. I exclude below 30 BMI because they don't even consider that "fat" somehow. And example would be someone 5'4" and 180-200lb. Middle fat is essentially "impaired mobility". Think BMI 35-50 depending on how they carry they fat. This is where you get most of the knee problems and huffing to walk across a room. An example would be someone 5'4" and 270lb.  Death fat/large fat is essentially "closer to immobile than mobile". Think BMI 60+. An example is someone 5'4" and 350lb.