If only my wife were thinner.
Are you serious? Why do you have a problem with your wife's weight?
She's so big, I don't like looking at her.
You should not criticize your wife's. It's rude to judge someone based on their appearance.
My wife knows she's fat, if she doesn't want to be fat then she should begin to diet.
That doesn't make it okay to criticize her for her weight. I think what you should do instead, if it really is that much of an issue, is start a short simple workout regimen and encourage her to join you. Working out together can be great for bonding and help you both be healthier.