What (implicit) option control translation "user" to "first lastname"?

[Johan Persson]

I have working postfix setup but there is one thing I don't quite
understand where it comes from (and that bothers me)
I only use one map which is a "sender_canonical" map to translate local
user to a valid external email address. This works fine.

However postfix also adds the full name (as taken from passwd) in front
and as far as I can see I have never told postfix to do this.

So I get the translation something along the lines of

"u...@localhost.nil" => "first lastname <some_em...@example.com>"

but I would have expected it to be

u...@localhost.nil => "<some_em...@example.com>"

After reading through "Postfix Address Rewriting" and the settings
I cannot really find any explicit setting that controls this.

So what option controls the fetching of the full name from the passwd 
file?

/J