Is it a sin for me to be a fat (overweight) wife?

A person is required to take care of his/her health. If someone is overweight to the point where this threatens his health, halacha requires him to do what is necesary to ensure good health. From your question, however, I understand that your concern is about a husband who is unhappy with his wife's appearance. The talmud sees a wife's attractiveness to her husband as a value. In a marriage, each spouse should, within limits, want to do what he/she can to make his/her partner happy. If a wife doesn't feel this naturally, it might be worthwhile for her to analyze why and perhaps consult a marriage counselor.