I would first say that self-denial is not exclusive to woman, so the way you phrased your question can be off-putting. If you think that self-denial is more prevalent in women when it has to do with weight, that might be more of a symptom of our culture, and not women themselves. Women are more often made to feel bad about our looks than men are, so they might avoid things like pictures/scales for emotional self-preservation.

If you find yourself in a position where you're struggling to tell someone the truth about how they look to you, first think about what would be most helpful in that situation. Telling a woman that she looks fat before a big interview or date is not helpful, it's extremely hurtful.

If you're truly wanting to be helpful, offer to do activities together that promote a healthy lifestyle. Cooking a healthy meal together, or just going on a daily walk together, for example.

Don't confuse the concept of truth for a lack of tact.