I found a very fat girlfriend. At first, I didn't care about her 155, 130 kg figure. I can't stand her figure more and more recently. I told her to lose weight.
In this way, don't say you don't like her fat, let her lose weight. Just tell her that obesity is not good for your health, and you will get high blood pressure, hyperlipidemia, diabetes and so on. You want her to be healthy. If she gets sick and then takes an injection to take medicine, you will feel very distressed. Say no once, say it several times. When she realizes that you really love her, her motivation comes.