I can't quite put my finger on it, but I don't think all NPCs are equal in terms of food and weight management. Could be down to some of the attributes, like strength as you said.
The Helm sisters. One is always optimal on weight, the other is always "slightly" underweight. I even tried to take her out to eat, give chocolates and extra rations and haven't been able to get her weight up. Even when both were pregnant, they didn't gain weight.
Astrid is another case. I need to keep her on low rations, otherwise she will always go up in weight. At one stage she was even dangerously overweight. Could be "natural" to her, or it could from her job (Golden Dragon). Haven't quite figured it out yet. Astrid has gone (in description at least) from normal weight to slightly overweight to dangerously overweight.
Most of my people are set on 3 rations (they actually only consume 2 due to the skill of the pantry or chef staff... can't remember which).
I'll be honest, I didn't pay attention to the "body type" text or even the weight (the number) itself. I just checked the "inventory" part where there's the weight description.