I've always been a slightly critical person AND don't really have a knack for pushing compliments I don't mean. Okay - I probably do, but a) honesty is easier and b) I want you to be honest with me.
Besides - what takes more time away from your life in a response to the following question "Do I look fat in this?"
Saying no takes way more energy because there's always the task of having to convince the fat person you're being sincere (nice)
Say yes and it's over. You're not doing your fat friend any favors letting them go out looking like they just won a pie-eating contest after lunch.
In the cases where your friend truly doesn't look fat you can say no and then rather than plead your case of sincerity simply remind them of all the times you didn't spare their feelings.
Them: Do I look fat in this?
Them: Are you just saying that?
You: Remember that time I told you that all you needed to do was put a ring through your nose and matadors will be poking you in the neck?
Them: . . . yeah.
You: I'm not here to spare your feelings. I'm here to be a good friend. Now go get me a cupcake as a reward.
ANYWAY - I got to give good ole Zahra some email advice today as to how she should do herself up for an upcoming wedding!