SQL & PL/SQL

Hi All,
How do you determine the PIPE size based on the message size?
e.g. I have a message that will be 17bytes length. If I put 100
messages in the pipe, theoretically it should be 1700 bytes. But
in the v$db_pipes, it shows about 6500 plus bytes.

Any ideas on this pipe math??

Thanx
Kiran