Well, given that 50% of women and 60% of men are overweight or obese, and that this is related to age, you could always leave her and try and find someone who is thinner into middle/older age?
There aren't many of those, though, statistically speaking.
I think trying to get them to be thinner, exercise more, eat different food is on a hiding to nothing, if they want to do it, they may, but it's unlikely and we also know that most people who lose weight put it back on, so significant and sustained weight loss from that starting point is highly unusual. Probably 2/3% of people can do it.
Now this thread will fill up with people saying 'but I'm thin' or 'but I did it' which may be true for them personally, but the battle of the bulge is statistically speaking mostly lost, so you can either come to terms with your partner as they are now, or leave them and find this person who is never going to put on weight and always respond when you suggest exercise/healthy diet and keep it off into old age.
Everyone else just gets on with what they have, which in the main, amongst my friends, is women and men who look nice but are a bit heavier than when younger and they and their partners would ideally like but values their partnership differently anyway, and just sees it as a part of ageing and loving them.
If you can't do that, and sexual attractiveness is very much tied up with being slimmer (as it is for some people, especially when young)- then you do have the option to get out, especially if your love life has died.