It's not really too bad, since even if you fail you still get a benefit. It is just smaller: +3 instead of +6 per action. And iirc there's a daily cap how much you can increase it.It would be nice if weight training had a lower ceiling for checks then, for characters who want to max weight right at the beginning.
Also, to clarify: you don't need weight training for character to get more plump. Weight training will train muscles and simply won't be detrimental to your goal of gaining fat. All you need for character to become more plump is to eat a few meals a day and avoid cardio actions.