If your wife is afraid she's fat, she does not want you to tell her that she's not - that would make you a liar. She also doesn't want you to try to be accepting - that would make you patronizing. Blending the two, however, takes her mind completely off the problem.
Me: "You're not fat, honey. And, besides, I like thick thighs."
I woke up in a half-nelson leg scissor lock...five times.