I want my wife to gain weight
Why do you want your wife to gain weight?
She's underweight and the doctor says she should gain weight.
It's okay to worry about her health. But you shouldn't force her to against her will.
I'm not telling her to gain weight, just making the food more convenient for her to eat.
If that's the case then I think you're doing the right thing by giving your wife the opportunity to gain weight and be healthier. It's important for partners to help one another lead healthier and happier lives.