"How she has grown!"
Of course there are other variations, plus you can dress it up with things like "My gosh" and so on, but structurally "how" works here. When something has occurred or changed over time that surprises you, an exclamatory with "how" usually works. It's basically like saying:
"The manner in which (= how) that has happened surprises me!"
Hope this helps.
Student or Learner