Being fat does not indicate gluttony. Many overweight people don't eat much more than a thinner person. The factor is exercise
Being skinny does not mean that a person is not a glutton. Their body may burn it off. But they are taking more than their fair share of food perhaps at meal time or when a guest a someone's house.
I have never heard of anyone being DF'd for gluttony. But then for that matter, I have known very few JWs that were DF'd for drunkenness.
Knowing whether a person is a glutton or has a medical condition, would mean the elders would have to pry into very personal matters. Since many elders I know are people of size, it might be a hypocritical exercsie. Then there are conditions like bulimia and aneroxia-bulimia.
Should JWs be DF'd because they don't take care of their diabetic condition properly?