[exim] Exiscan: Skip spamassassin checks on big messages

I'm using Exiscan + SpamAssassin. I'd like to skip the SpamAssassin checks
on large messages (defined here as anything bigger than 1 megabyte). I have
the following configuration in my data ACL, but it doesn't seem to work;
spamd still gets called on messages larger than 1 meg. What am I
overlooking?

  accept
       condition = ${if >{$message_size}{1M}{1}{0}}
       log_message = Large message; skipping spam check.

  warn message = X-Spam-Score: $spam_score
       spam    = nobody:true
  warn message = X-Spam-Report: $spam_report
       spam    = nobody:true

  deny message     = Rejected this message because it looks like spam.\n\
                     SpamAssassin's report:\n\
                     $spam_report
       log_message = Rejected message as spam.  Score was $spam_score.
       spam        = nobody:true
       condition   = ${if >{$spam_score_int}{70}{1}{0}}

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David Brodbeck, System Administrator
InterClean Equipment, Inc.
3939 Bestech Drive Suite B
Ypsilanti, MI 48197
(734) 975-2967 x221
(734) 975-1646 (fax)