[exim] message-size condition doesn't work

Hello,

at
http://www.exim.org/exim-html-4.60/doc/html/spec.html/ch40.html#SECTscanspamass
there is the following condition:

deny message = This message was classified as SPAM
      condition = ${if < {$message_size}{10K}}
      spam = nobody

I'm using this, but I noticed, that exim passes even messages with >
300KB and > 1MB to the spamfilter. Why doesn't it work as expected? My
configure-file looks almost like the default in this area.

Regards
Marten