That moral theology book sounds kind of odd...
CE entry on Gluttony
Gluttony is in general a venial sin in so far forth as it is an undue indulgence in a thing which is in itself neither good nor bad. Of course it is obvious that a different estimate would have to be given of one so wedded to the pleasures of the table as to absolutely and without qualification live merely to eat and drink, so minded as to be of the number of those, described by the Apostle St. Paul, "whose god is their belly" (Phil., iii, 19). Such a one would be guilty of mortal sin. Likewise a person who, by excesses in eating and drinking, would have greatly impaired his health, or unfitted himself for duties for the performance of which he has a grave obligation, would be justly chargeable with mortal sin.
I remember one Saint, I think it was St. Anthony of Padua, that used to drive his mother nuts. She would say, "What would you like to eat?", and he would always respond, "Whatever it pleases you to serve me."
Now, I am "gravitationally challenged" somewhat, so I would like to point out that not every fat person is in a state of mortal sin by being fat. Many of us have office jobs (another sign of a decadent society), so even if we eat normal portions, we will be a bit hefty (though my doctor yells at me to exercise and lose weight, and I promptly ignore him and light up another cigarette).
Also, there's the parent curse. Your kid doesn't finish his or her plate, and we can't let food go to waste, can we? *sigh*