You have two options: 1) leave her; or 2) she needs to lose weight.
You are obviously in favor of option 2 at the moment. My suggestion is to not beat around the bush. Be direct but consider her emotions. She is probably depressed. Of that I am certain. There are two ways, in my mind, to approach goal 2.
Approach 1: concern for her.
Tell her you love her and that you are concerned. Tell her she has gained 75 pounds and that is not healthy, physically or mentally and you want to know what's going on and that something needs to change today. The change can be small but progress must be made.
For example, you can choose 1 day of the week as couples exercise activity day. And one day of the week where it is salads only day. Or make a water or unsweetened tea only rule for beverages (except maybe lightly sweetened coffee). Be creative.
Then add one more thing every couple weeks. Suggest adding another day of exercise or salads because it's fun. While you engage in those activities tell her how happy it makes you. And encourage her but don't judge her.
Approach 2: concern for yourself.
Tell her you don't want to be mediocre and that you want to live life to it's fullest potential in every facet including health and fitness. Tell her that all the resources to be a fit young active couple are available and that you want to be one of those couples for no reason other that you can.
Mention that muscle is easily gained when you are young but almost impossible to gain as you get older. And that muscle is retained well into old age (queue pictures of Arnold). So you want to take advantage if that as well.
Anyway, what I did with my girl is tell her straight up at the beginning of our relationship that I would leave her if she gained weight. My reasons were that I find people who gain weight lazy. And that I don't date lazy people. Worked for me. Now she actually enjoys working out independent of my influence. In fact she looks down on people that not only are fat, but don't workout.