Probably at first, just be very complimentary of her figure and body. Not just with words but with actions and your hands. A lot of girls love their body being rubbed lovingly. At some point the topic of weight will come up and if she talks about “I should lose weight” you can just very honestly tell her you’ll miss her curvy softness - or something along those lines.
It’s generally a process that happens over time - especially if she’s been criticized by others. People typically need time to process and reconcile an idea one step at a time. Be patient with her, tell her more as she’s ready to hear it, which might not be as fast as you want to tell her. Communication and honesty are important. You could do a little fact finding here or there as opportunities present themselves. Maybe an article you read or news story can spark a conversation that isn’t about her weight, but gets her sharing opinions or feelings about chubby people or weight loss or something. You might be pleasantly surprised.
Don’t be afraid to offer her seconds or playfully tease her for not finishing the last bite. Nothing intense of course. Don’t make her feel obligated or self conscious. Down the road, when you tell her you like her chubby and would enjoy her getting bigger, she’ll be able to think back and the fact that you never encourage her to stop or diet and have always offered more. That will help her see you are being genuine and not just saying it to be nice.
Just some ideas. Think about her personality and how she responds to things and that should also help you know what to say and when to say it. You want her to feel loved and accepted, not like she’s flawed or something for being smaller than you’d like
2 years