There is one question that simply isn't fair no matter who asks it. No matter what the answer, you are either in trouble or assumed to be lying.
So what can you do? Chances are you don't think she is fat. However, getting her to believe that and understand how you feel is a difficult task.
The solution, believe it or not, is simply not to try. She isn't looking for you to convince her she isn't fat. She is looking for you to connect with her and acknowledge her fear. You don't need to confirm or deny it.
Women are shown, over and over, an unrealistic idea of what some people consider to be beauty. The image they see is neither realistic, nor beautiful. To them, these images are beaten in until they become some horrible, twisted, half-truths.
So when she asks if something makes her look fat, tell her it doesn't. Explain to her something you know to be true: no matter what someone else sees as beautiful, she is the most beautiful person in the world to you. You wouldn't be with her if it were otherwise. Then, if she isn't convinced, listen to what she wants to do to feel better; ask her what is bothering her specifically and really listen to her response.
Don't rush her response and don't be dismissive. Remember, to her, this is important to how she feels. Your opinions don't matter now. You may feel that she is completely wrong, but if you speak up now, you will never be right. She needs you to hear her out. Once you have, and once she is done, have her set a goal. Now, commit yourself to helping her work towards whatever goal she sets.
While struggling through this giant mine field, remember your English classes and the lectures on arguing a point. You must always be willing to acknowledge valid points made by the other side, even if you don't like them.
-Accept what she feels as a fact. Not that it is true, but the fact that she feels that way and that it is valid for her to have her own opinion.
-Strive to understand why she feels that way.
-Explain your own opinion, remembering that it is no more valid than hers.
-Find common ground. What can you both agree on?
-Seek a common goal. What do you both find realistic that is in line with her desire to eliminate her insecurities and also concurrs with your opinions?
Once you have aligned yourself with her, shown that you are supportive in helping her achieve what will make her happy, you will have a chance to talk about what you think of her feelings. She has to know, first and foremost, that you are willing to listen to her and understand her viewpoint. Only then can she accept that there may be another valid opinion.
