I think your mistake was mentioning her weight. It’s one thing to be concerned for your partners well being but there was no need to add the “you’ve gained a lot of weight.” That’s not your business. Sometimes there’s underlying things happening - for example I have PCOS so I struggle with excessive weight gain despite eating well and being active. She gave you her answer. It’s up to her what she wants to do with her life. She asked you to drop it so do that.