I think my wife needs to lose weight
Why do you think your wife needs to lose weight?
I'm worried about her health.
It's understandable to care for your partner and want them to be healthy. However, you really need to approach the situation tactfully, and perhaps take steps to get healthier together.
Fuck that, I'm going to make her get thin.
You should not try and force something on your wife. It could damage your relationship and the only way people actually commit to making more healthy choices in their life is when they have decided it's what's right for them.