Situation 1: If this girl is not fat at all. Don't be surprised, girls are not fat and say they are really fat. You can say that your figure is very standard, alas, it seems possible, a few pounds thinner than the average girl.
Situation 2: She is really fat. There's nothing you can do about it, but don't agree with her that you are really fat. You can say that the status quo makes you look cute, but you must be practical, otherwise you look hypocritical and she doesn't feel comfortable listening. In addition, it can be said that she has a good foundation, and if she can lose weight persistently, she will be a stunning beauty. This is easier to accept.
Situation 3: She is just a little fat. This kind of woman is generally more concerned about her appearance and what others think of her, so you should directly deny it, but don't say that she is not fat at all, but she is not obvious at all. Or if the opportunity is right, you can say: Have you lost weight in any way recently? Looks thinner than before.
If she continues to be modest, you can say that it is not good to be too thin, and it is wiser to be full. If she still says she is fat, you will also say that she is really close to the standard figure. Just work hard.
Or: the face is still so small, but the chest seems a little big. She didn't answer the girl's question directly, but answered the girl's question in a decomposed way. Why do girls ask you if she is fat? Because she thinks she is fat, but! Even if a girl shouts "I am fat again" every day, she wants to hear others say "you are not fat" and "you have a good figure".
This answer not only admits that girls are a little fatter, but also praises one sentence. Even if it's not true, girls will be happy to hear it. You grow an inch taller, and I love you an inch more. This is definitely true love. True love will not abandon each other, no matter what the other person's weight becomes, love will continue as always.