This seems strange to me, there’s got to be more to the story. This articles smell like funny like the Accord Study. I guess its probably where you store the body fat. If its stored as subcutaneous fat around the body, not really much of a concern as subcutaneous fat is rather benign and increases insulin sensitivity, and when appropriate correlates with good health and longevity. But if the fat you store goes to the belly as visceral fat, that can be cause for concern as this type of fat is pro inflammatory leading to increased risks for a variety of health complications and insulin resistance.
If you are on a low carb/low insulin regime you are not going to have allot of body fat or a high BMI in general compared to those on a higher carb/higher insulin regime. You wont be storing as much fat as you’re more likely to metabolize it in the presence of low insulin levels. Also on lower carb diets you won’t be storing as much your energy as glycogen. On a higher carb diet allot of your weight comes from extra muscle glycogen from carbohydrates, extra body fluid packaged with this muscle glycogen, general extra body fluid as insulin causes sodium/fluid retention, and of course extra body fat from surplus carbohydrates not burned or stored as glycogen.
Larger people have more tissue reserves of protein and fat to draw off of in times of need and the skinnier people may have had other factors complicating the disease. If I had to put my money on somebody with a BMI of 25 with a HbA1c of 7.5 and somebody with a BMI of 20 and HbA1c of 5.3, my money would go on the 5.3 anytime.