I think this rule is enforced unevenly. You might get a person that is very fat ("very fat" is not able to fit in one chair at the Kingdumb Hell) getting along just fine, until they get a hounder-hounder that orders this person to lose weight. I have seen a few that were very fat (including one that was a pio-sneer, despite being so fat that she needed a walker to waddle into the Kingdumb Hell and couldn't sit in one chair).
Problem is, the religion makes it very difficult to maintain a healthy weight. One way to lose weight and keep it off includes walking a whole hour a day. Walking does not mean getting in and out of the car, walking or waddling to the door, and stopping. Walking means going at a brisk pace, where the person needs to consciously exert themselves, for a whole hour. Try doing that, and see how long it takes Brother Hounder to tell you that, if you can walk a whole hour a day, you can go out in field circus that hour.
Other things that the religion does that makes keeping your weight down impossible include the pio-sneer diet, not getting enough sleep, the fumes from suit dry cleanings (which are stored in fat, making it very difficult to lose that fat), sitting 2 hours at a time at a boasting session, stress, and the continual rush to hurry up and get out in field circus. These things make losing weight needlessly difficult, independent on calorie intake. And, with all the stagnation the witlesses endure, eating is often the only thing left.
And there are those who are fat, not because they are lazy pigs, but because they are sick. Numerous things are known or highly suspected of causing this. Eating excess monosodium glutamate and/or aspartame, especially in small children, makes you fat and this fat is extremely difficult to diet or exercise off. Chlorinated sugar and fatty acids (think: Splenda, chlorine in the water reacting with fat in your diet) both mess up your liver. High fructose corn syrup also ruins your liver, making keeping weight off difficult. Fluorides in the water ruin your thyroid (and pineal gland), slowing your metabolism. Soy and canola (rape) oils slow your thyroid (coconut oil speeds it up). People deficient in iodine (which you probably could use much more than you think you do) have trouble with their weight. And, you can get fat by dieting--your metabolism is messed up that way. Most very fat people actually eat less than healthy people, but their metabolisms are impaired. Which is why I don't think being fat is a sole criteria for gluttony.
Trouble is, the hounders are the ones that make up these rules. Even when dieting is why you are fat (it ruined your metabolism), they will order you to give it yet another chance. They lose a few pounds, can't take it, go back to normal (which is actually normal eating for everyone else), gain back the few pounds they lost with interest, and get more of the guilt and pressure that makes it even harder to lose weight in the first place. Yet, if they start exercising and cleansing the poisons out of their system, the hounders nag them about taking care of their health to excess. And, either way, you could be running into "brazen conduct" if they hound you about it and you still do not obey.