>"No, it doesn't make you look fat" feels like a better response than something else which would cause her to spiral.
And you're right. It'll feel like the better response; because all of our social circuitry heuristics honed over our lives by a biological imperative to minimize energy expenditure will happily present that as a quick, harmony preserving fix. However, in that case, are you not doing an injustice to your partner with self-image issues by A) bearing false witness to your true feelings on the matter (dress does not fo you any favors, darling) and B) not addressing the pathological denigratory self-image with active confrontation and refutation of the validity thereof, with an accurate accounting of your own reasoning?
Example: I've had that question asked of me while my partnner and I were trying to shopping for wear to attend a wedding in. My partner loves blue. Like BSOD blue. Cobalt electric blue. She was drawn to a dress in that color. She had a friend along that was already giving the customary platitudes, but asked me my thoughts on it. That color, unfortunately, in my eyes, does not love her back. Her skin/complexion tends toward the rosey and warm; a combo which strongly clashes with and fights with said the coolness of the blue, and which would tend to cause the eye to pick out the paler aspects of her instead of accentuating the her livelier parts. The cut, and just how it laid on her body didn't complement her. It hid her best parts, and accentuated the parts I knew she was ultimately more self-conscious about because there wasn't a damn thing to do about them, and the baggage that came with it. The strong blue just visually overwhelmed the visual experience to the point where all you saw was the dress, and how it dominated the image of her; not the person wearing it, with their actions and body being accentuated and emphasized by the dress. I told her if that was what she really wanted, and if it made her feel good, then what does it matter? However, I thought she'd look better either in a purple, or perhaps more importantly a dress of a different cut. One that instead of looking like a wrapper, would actually lay on her body in a way that would naturally draw the eye to her, and what she was doing.
End result? She kept looking. She ended up picking out a pattern and cut that she felt was more befitting for her role/relation to the two getting married (deep or high saturation colors are apparently reserved for the bridesmaids per her thoughts on the subject, and something a bit more matronly made her feel more comfortable at the event). She also picked up a dress I thought really made her look gorgeous, and I adore seeing her looking regal af in, and wouldn't be out of place at about any event.
I acknowledged, and reaffirmed that ultimately, the most important thing was how she felt. I also provided a truthful evaluation about the answer to the question, giving her more insight about how I see things,the things I look at, and the affect they have on me; and as it turned out, she's gotten way more use out of the dress she ultimately picked than she'd ever have been comfortable getting out of the blue one. So by taking the path of truth, I opened a door to a novel experience for her, and she says she's happier for having done so.
Not to mention, you're kind of doing yourself an injustice by not really valuing your own feelings on the matter. I'm not saying that it's okay to tell someone the dress looks them look like they're wearing a trash bag; but it's absolutely okay to express your feelings on something in that type of circumstance, even if your rational side tells you that maybe you should be silent. The only way to help someone through the healing process is to acknowledge there is something there, and to be supportive and truthful. Placing care about everyone else above your own care for yourself is as sure a recipe for a bad time as any. One can't wholely and truly give for others what one does not yet possess for themselves have.
It may not necessarily win one many invites to fancy parties, but I've found that getting into the habit of twisting things makes the process of growth, healing, or recovery so ruinously complicated as to present an overall harm in comparison to the brief cauterizing application of truthful, sincere communication. See the cleverest code as compared to the most straightforward as applied to social dynamics. Many of the same principles transfer.