People think this is a nice thing to say and I'm sure it is for most people. I'm sure it happens all the time that a woman feels fat from a recent weight gain, but feels reassured by others that they can't tell.
In my twisted mind, the statement, "You don't look like you've gained weight," sounds like "You've always strained your clothes like this and had that double chin."
Recently I responded to this reassuring statement by saying, "Please don't say that. It makes me feel bad. These days I don't respond well to anything but I'm sorry to hear that. " The person smiled and said, "Okay, I'm sorry to hear that. But I really can't tell you've gained weight!"
She thought she was being nice and I can't blame her. To normal people, that probably is nice, but I felt like she was insulting me after I'd asked her not to.
Yeah, I'm recovering from depression again. One way I cope is to ask people specifically for what I need. It doesn't always work.