Arkm:
It is not about beeing a chicken it is about how to go for it the right way because i know it is a negative theme in her past
That's fair.
But yeah, just talk to her. This may take several conversations, but start off neutrally. Very casually, let her know that you love her size and that you wouldn't mind if she got bigger.
If she response well to that, let her know that you find it a turn on. But make it very clear she has the right to refuse. Even more so that you will respect her decision and won't bring it up again if she doesn't want to talk about it. Also, make it clear you will still love her and find her sexy if she lost weight.
Maybe she will be open to it. Maybe not. But if you are respectful and keep the lines of communication open, you should be fine.