I do not think you should tell your wife that she let herself go after your marriage and she needs to lose weight. There is no polite way to ever say this. First of all, have you let yourself go? How much weight have you gained since you got married? Do you set a good example for your wife in your marriage or do you insist she cook meat and potatoes for you every night?
There are maybe some subtle ways you can help her to lose weight such as asking her if she would like to go for a walk with you. Or, cooking dinner for her for a change. Make her a nice big salad with lots of fresh vegetables. When she wants to order a pizza delivery, suggest something else. She will probably catch on to what you’re trying to do and ask you outright if you thought she was getting too fat. At that point you should respond nicely that you’re a little bit worried about her health and you just want to help her