If she is obviously overweight and she is self-conscious about it, just by saying 'let's go to gym together' can make her feel bad.
I am meeting this woman who wears size 2 but got some belly. I just told her 'you need to go to gym to get rid of this' by pinching her belly. of course she got little upset but got a gym membership

I could do this way cuz obviously, she is not fat in average person's view.
I persuaded her by saying 'this would be good for you whether we are together or not, wouldn't you be happier if I go to gym and get better body'
If you are fit guy and she is kinda overweight person, it is fair for you to ask that.
If she wouldn't care if you sit around the house and get fat like her, then I don't know what to say
