That's not the way a relationship works, sorry (honestly). She does not owe you a bangin' body, just like you don't owe her finding her weight gain attractive. The only thing you can do (and there's no guarantee at all for this to change things for the better) is to address the issue calmly. Avoid points like "I stay in shape so why can't you", "why don't you see this yourself", "I'm worried about the relationship" - all of these will backfire. Tell her you've noticed her weight gain, ask if there's a reason for this - stress, hormonal issues, more movie nights with the girls, whatever - and if she is concerned herself.
Here's the deal. To many people who gain weight uncontrollably, it's like getting more hair growth on your back - it seems like there's nothing you can do about it and it embarasses them. If you approach this with a lack of sensitivity, you will be the asshole for making her feel bad about herself and criticising something that's none of your business, that can't be helped due to her metabolism/thyroid/etc. (confirmation bias, there are plenty of people online saying that weight can't be controlled and she may believe them) and that shouldn't bother you since curves are beautiful.
Now, that's the worst case scenario. She may open up and ask you for fitness advice. Chances are, however, that she's unhappy with it herself and won't thank you for rubbing it in.