Could be a configuration on the mail server (out of your hands, unless you're the admin). According to JavaMail API docs:
When the headers of a POP3 message are accessed, the POP3 provider uses the TOP command to fetch all headers, which are then cached. Use of the TOP command can be disabled with the mail.pop3.disabletop property, in which case the entire message content is fetched with the RETR command.