IN addition to what Omega said, I have a couple of other questions - not necessarily that you need to answer them here unless you want to, but some things for you to think about.
You say she eats as much as you or tries to match you, but since she was significantly overweight before, it sounds to me that this might be unconscious rather than conscious. I know that when I was eating with my ex it was easy for me to "justify" in my own mind eating more - not consciously .. just that I'd fix 2 plates with equal amounts of food. One thing that helped me was using smaller plates to eat off of - I couldn't load them as full and then I had to make a conscious decision to go back for 2nds ... and when I realized I was eating 2nd helpings, it was easier to stop.
When you eat together, what kinds of meals do you eat? Are the meals you fix healthy, or not? Again, my ex was raised in a "meat and potatoes" household and for him a meal used to be a large serving of starch, a large portion of meat, and some veggies on the side. Usually served with a couple of slices of bread and butter. When we separated, I started eating veggies and meat with starch as the smaller side and just that alone made a huge difference. I hadn't realized how his way of eating - and cooking to make him happy - influenced how I ate. Could this be something you and your wife do and could start making some changes in?
Also you said that *she* starts exercise programs but doesn't follow through - do you ever exercise together? Go out for a walk or a run, go to the gym together, etc? It was always very hard for me to get motivated to go to the gym and exercise when my husband would come home every night and sit on the sofa and veg out. It was very lonely for me to go do these things by myself and even though he encouraged me verbally, I felt like support was lacking because he wasn't out there doing something with me.
I also have to point out that you said in your post "I have sat down and talked to her a couple of times" ... and would like to give you a perspective on that. My husband was fairly athletic and in good shape and didn't have to put a whole lot of visible effort into being fit and slim. Me? I look at food and gain weight. Plus I was never encouraged to be athletic as a kid or young adult. So already I felt at a disadvantage. A couple of times my husband tried to "talk to me" about my weight and ... I'll be honest, even though I know his concerns were about my health (I had heartburn every night, I had no energy, etc. etc.), I was VERY RESENTFUL of what I saw as a holier-than-thou attitude. He honestly had NO idea how hard it was for me to lose weight, to be motivated, to feel like it was something I could do - especially when it was so easy for him. All he had to do was stop drinking Coke for 2 weeks and he'd drop 10-15 lbs. Being "talked to" is quite possibly the WORST thing you can do to help encourage your wife ... it is very likely to make her feel both resentful and think that you find her unattractive or even repulsive. It becomes a cycle that is hard to break.
My suggestion would be if you want your wife to become more interested in her health and weight, that you should be interested in YOURS and invite her to participate. Start working out and ask her to come with you to keep you company - that it's lonely for you by yourself (even if it's not and even if you think she might slow you down or hold you back). Tell her that YOU want to start eating healthier and ask her if she'd be willing to experiment WITH you in different kinds of cooking and eating. Make this something that YOU want to do and you want her to be a part of, not something that you want HER to do to make you happy.
I know it's hard and I do get the impression that your heart is in the right place ... but I also know how hard it is to be the overweight wife of a man who has no problem losing weight and staying in shape. It's very difficult not to be resentful and give up before you've begun.
And ultimately anything that she does will have to be her choice. You can encourage and invite all you want, but she has to make the decision for herself.