Sometimes you've to be smart and choose your battles for your own sanity when interacting with a woman longer term. Just as you choose which arguments to ignore and in which ones to make your stand so to speak.
Any comment that could be perceived as negative about her dress and i'll promise you, you'll hear her talk about that for the next few months to years. Hardly worth it. Again, we're fundamentally different. As a man you appreciate and expect an honest answer when you ask a question, it's ingrained in you as your best chance of survival in the tribe. Women don't want to hear the truth, they just want to hear what makes them feel good. Big difference.
Most of the time their questions are just noise. They run around anxious 24/7 and want to feel sexy for you, so when she does put in the effort giving her your validation is just a smart way to reinforce more of that behavior.
I'd never encourage saying that she looks great when i don't think so though, in this case a covert comment like EyeBRollin suggested is the way to go and then deal with the overweight problem later on by encouraging her to join me in the gym because i find her sooo sexy dripping in sweat.
