My SIL had a breast-reduction surgery; She mentioned it to me (we would be visiting during them the recovery period). But when I saw her, I really didn't notice anything.
She mentioned it, didn't I notice?
I said, "You look great--but to be honest, you look exactly the way you've always looked in my mind. I've always seen you this way, looking this great."
That might be the thing to say to Barb: "You look great! But you've always looked this great in my mental picture of you. I've always thought of you as slim. I guess it's what your personality has always been to me."
The same as when people say, "You look nice today." The feeling I get is, 'So I don't look nice on other days?'
I've taken to saying, "You look particularly nice today!" or "you look extra nice." I even sometimes say, "You always look good, but today there's something special."
But I also think it's really unfair to "hear" the message "you don't look nice on other days."