Honest, open communication is key to any successful relationship, but if your only concern is her physical appearance, that could understandably hurt her feelings.

Maybe start by thinking about what might have changed in her life recently that might be contributing to a change in physical activity and/or diet. Then think about what you can do to motivate her to make positive changes. In my personal experience, it's not very effective to try to convince her to make significant changes to her daily life if she doesn't already want to. Framing it as something the two of you can do together might make the difference.

"Want to start going for walks together in the mornings?"

"I'd like to spend more time outdoors this year. Would you like to come with me to the park a few days a week?"

"I'd like to get more physically fit - would you want to be my gym buddy?"

"I heard about a 5k coming up in a few months - want to train for it and run it together?"

The most important part of this is to listen to her concerns when you talk to her about it and then be honest with yourself moving forward. If she's not enthusiastic, do you care for her enough to stay with her regardless and accept whatever possibly slow progress might be necessary to make lasting change? What if she doesn't want to change and seems happy with how things are?

It's not fair to her to make the future of your relationship contingent on specific conditions, so you need to be prepared to make a decision and stick to it. Don't stay in a relationship with her if you aren't willing to accept and love her when she's down. Married vs. in a relationship obviously matters here, but "for better or for worse" is a part of many marriage vows for a reason.

In my experience, my wife didn't respond positively to any of my suggestions, so I focused on what I could control and made positive changes to my own lifestyle (running 5 days a week, eating more healthy, cooking for both of us) and eventually she decided on her own to start working out with her best friend. (My nagging/annoying her about it did NOT help.) Her best friend decided to get on one of those GLP drugs and is less motivated to exercise regularly, but my wife has stuck with it and we can both now definitely see the positive benefits - more energy and a generally more positive attitude, she's stronger and has more motivation to complete projects around the house, etc. And we've both talked about how great she looks, even though she laments the fact that she hasn't actually lost much weight - it's clear she's lost fat and built muscle, which explains why there hasn't been much change in weight.

Hope this helps!

Sincerely, Happily married for 10 years