My (M61) wife (F61) has lost all her weight, she has gone from a size 4 to a size 1. So how do I tell her she doesn't look as good?

My (M61) wife (F61) has changed her diet and she has lost all her weight, she has gone from a size 4 to a size 1. I don't like how loose the skin around her face looks. She had a facelift last year and I was really happy that it was so successful that it made her look 10 years younger, and now it's like all that progress was lost. I'm not too bother about other parts of her body. She was never big to begin with, so it's all in the facial area for me, and she was so happy after the surgery and is still happy now, so I don't want to hurt her feelings. But I believe it's fair that she should know how I feel. She watches what she eats and works out, but how do I express this to her without hurting her feelings?