You can either wait for the perfect conditions to present themselves or you can create those conditions for her. Waiting for the right moment is too spontaneous and can cause things to go awry quickly. Here are some suggestions:
Have her spend a day at the spa.
Take her out to dinner at her favorite restaurant.
Send her on a shopping spree to the mall with her girlfriends.
Once she's had the best day of her life, move on to the next step:
Step 2: Do what she wants to do
After her amazing day (courtesy, you) she'll be in a tired state of elation. Tonight, do what she wants to do. Here's a few more suggestions:
Take a slow evening walk through the local nature trail. Make sure you're holding her hand the entire time. Enjoy the sound of the crickets and your feet crunching along the gravel.
Snuggle up on the couch and watch her favorite chick flick (you secretly like A Walk to Remember anyway).
Take a leisurely drive out to "your spot" — that place where you first kissed her, where you first asked her out or where you both sat together counting shooting stars.
Take your time with this step. The big moment is coming up. When you've got her right where you want her, continue on to step three.
NEVER tell a woman she's fat! Step three is to NOT tell her. That's a one-way ticket to getting emotionally, physically and even socially blacklisted (Women talk, and if you even want to be able to flirt with Margaret from Wednesday Scrabble in 70 years, you'd better keep your mouth shut now).
Do you think you're the only participant in this relationship that's noticed she's gained a few pounds? She's well aware of her own body and doesn't need you to remind her. It's an amateur mistake — one you do not want to make, even once.
Did you think suggesting you both get a gym membership was passive aggressive enough for her not to notice? False! She knows exactly what you're getting at.
"We should start eating healthier"? To her, this roughly translates to "Stop gnawing on the butter sticks."
Accept it, man. No matter the situation, telling a woman she's fat is a bad decision.
Because you've made it this far, here's the real step three:
The real step 3: Tell her she's beautiful
Shower her with sincere compliments — the kind she used to dream a man would one day tell her. Be sincere about it. Tell her what you love about her. Tell her how beautiful she is. Tell her you love how she makes you laugh, and how you just can't stop thinking about her.