As a serious suggestion, if she doesn't ask, don't say anything. If she does ask that proverbial no win question "Do you think I am getting fat (or I got fat)", my answer would be "I don't know, what do you think? Do you think you have gotten fatter?", then go with her answer. If she says "Yes", then go with "Maybe a little, but not too much". If she says "No", then say "I agree". Or try "I don't know if you have gotten fatter but maybe you have lost some muscle tone because we have been so inactive for a while. Maybe if we try working out you can tone those muscles back up."
This is a very delicate situation. She is asking but she already knows the answer. She is just looking for some support from you. Men and women communicate differently. When men ask a question about themselves, they want a straight, no nonsense answer. When women ask a question about themselves, they are looking for support and maybe a little empathy.
Avoid giving a man's answer to a women's question.
Advice Needed: How do you tell her she's getting fat?
