Karyn decided to have fun with Jon. Nothing that would hurt him, and nothing that couldn't be reversed. Karyn knew that Jon had a fascination with girls who had very large breasts, so she decided to make a wish based on that.
"I wish," she said, "that Jon had boobs the size of basketballs. Nothing else about him would change. In addition, Jon would not wear bras or other breast support at all." There; that should teach Jon what it was like to go through life with the kind of knockers he found attractive.