Let's say they gained some weight, and they ask whether or not you think they are fat. You noticed that they have put on a few pounds, would you agree that they have gained weight or would you lie about it?
The point of S. O. is to find the ONE person in this special-snowflake society whom you can trust to share everything with, including the truth.
The instant you feel you must hide how you truly feel from your S. O. your "relationship" is either doomed or confirmed as shallow and false.
That said, try to keep your partner's feelings in mind and say it as nicely as possible. Honesty and politeness are not necessarily opposites, there's such a thing as constructive criticism; but if even that is too much for you to handle consider seeking a babysitter rather than a significant other.
If someone asks me a question with all assumptions that I will respond back in a lie, then I'd think something is seriously not right here. I'd tell them the truth! I wouldn't blatantly say they are fat, that's a harsh word. But I'd say yes, "You gained some weigh". I'd also respond by letting them know how I think the extra weight looks on them.
Don't ask for the truth! If you're not ready for it.
My wife and I have an agreement, if either of us thinks the other has gained a noticeable amount of weight, we are to call each other on it and get back in shape. We've both played that card in the past and gotten back in check. Married 10 years now and I'm in better shape than when we got married. She's gonna need to be told again soon...
If she had gained enough weight for me to lose attraction to her, and if SHE was the one who brought it up - i would be honest and tell her that i noticed she gained weight. I wouldn't call her fat, but i would encourage her to lose the weight again and help her in any way i can.
I think if both partners started the relationship , or had a point in the relationship where being fit and healthy was something they both did and one of the participants in said relationship gave up the fitness. Then it makes sense for their significant other to tell them it's time to get back to the gym.
I wouldn't lie to him about it but I wouldn't call him fat. My boyfriend actually has put on quite a bit of weight since we began dating. I've acknowledged it when he's brought it up and when he talks about wanting to lose those extra pounds I encourage him in it.
Honesty, even brutal honesty, is better than lies.
I would try to be nice about though. "Hey, I noticed you've been gaining a few pounds, maybe we should go biking or to the gym together" As opposed to: "Holy shit, drop some pounds because your man tits are D cups and I'm afraid you're gonna crush me when we have sex. Nevermind, I can't get turned on with how you look now."