Don't surprise your parents with it. You don't have to come out and say she's big, but maybe send a pic of you and her together so they get the idea. They won't want to be embarrassed, so they'll know to ensure the chairs are sturdy, etc. Is she sleeping over? Have a backup plan in case the bed won't hold her.
My wife was 200lb when she met my family (chubby), but over the next decade grew to 340lb with mobility issues. My family adapted -- they knew to let her have the loveseat, and the dinner chair at the end of the table for less maneuvering. Your family will adapt to make it work if they love you.