She shrunk them herself.
She convinced Harry to do it for her.
What? She never got smaller front teeth...
She was jinxed, which made her teeth grow. When Madam Pomfrey shrunk them, she let her go on.
He knew that Percy talked about him all the time.
He knew that he had been Head Boy.
From what Ron said about him, he sounded like a prat.
He wasn't surprised.