I'm going to tell my girlfriend to lose weight if she wants to keep dating me. Do you think that's too mean?
You have a problem with her being overweight?
Yes, I like her more when she's thinner
Forcing someone to make a choice is going to cause them to possibly seek a different partner.. An ultimatum can lead to two partners forming resentment towards one another.
I just want my girl to be with me, we can work out the rest later.
Maybe you can cook healthier meals or plan ways to exercise together if she wants to but just flat out saying that to her is rude and uncalled for.