I used to feel bad about doing this, but you should never feel bad about expecting a thin woman.
Like other posters said, a woman that gains weight is disrespecting you. Period. Now, if she was fat and you still pursued her that,s different. But she must know when you enter into a relationship that abnormal weight gain is unacceptable to you.
My current girlfriend could lose about 10-15 pounds, but she's still cute. She's actually lost a few pounds since I got with her. I just got her working out about 2 weeks ago. Some tips:
-Get her interest level to the maximum (she must be in LOVE). Without love, nothing you do or say will matter in this arena. Once this is established, continue with the next few points:
-stop taking her out to dinner. Cook with her at home. Healthy meals. Lots of fruits and vegetables
-Reduce the alcohol consumption
-offer to go to the gym with her. If there is a cheapo gym near her (one of the $10 / month chains), sign up with her.
-Give her positive reinforcement and rewards for going to the gym. Do this every time. Compliment her gym attire. Tell her she's beautiful. Do this only after she works out / eats healthy. She'll do it more often
-Don't call her fat or allude to it. Only positive reinforcement will work.
-Do action dates only until the weight problem is solved. Think of calorie burning dates (trampoline park, sports at the park, roller skating, etc.)
Rinse and repeat.
Attention is always your ace in the hole. Withdraw if she gets back into her pity, complacency, and fat habits.