[exim] smtp_command missing stuff

I just connected to my mail server and manually sent a mail from of
the format:

MAIL FROM:<exim-users@???> SIZE=12345

The value of $smtp_command was logged. It was:

MAIL FROM:<exim-users@???>

I thought $smtp_command would have contained the SIZE too? I'm using
v4.63 at the moment...

Mike