It's a fairly common complaint from women, especially as they began to age.
The best way to reassure them, in my humble opinion, is to get them to accentuate their positives. It will involve many things about her; personality, sense of humor, looks, eyes, smile are the most obvious examples.
You, however, want to encourage her to dress in ways that flatter her curves. The better she looks, the better she will feel about herself. She can dress in ways to show her best charms, and draw the eye away from areas she doesn't feel good about.
If your wife has big(ger) titties, the necklines on her clothes should compliment them. Ditto when using a bra for effect.
If her behind is her best asset, having jeans/pants that fit properly is the biggest help. If she needs to tailor them for best fit, it's relatively inexpensive to have done, and makes a world of difference.
You will also benefit from this just as much; seeing other men ogle your wife as she walks by or trying to sneak 'peeks' are huge turn-ons and can lead to even more chances to be outgoing and flirty.....
I think this is a pretty good way to start, and you can also add to this as quickly as seems practical.
Shopping for lingerie together should be enjoyable for you both. Perhaps beginning to walk/exercise together for some tone is another way.
Hope this gives you some food for thought.
Best wishes!
