[exim] Exim4 skips SpamAssassin if attachments are too big

Hello everybody,

I am using Exim4 (version: 4.80-7+deb7u5) on a Debian system with
SpamAssassin. Everything works very fine except the case if an e-mail
has big attachments. Unfortunately it is quite common that lottery
spammers attach a pdf file which has 700 kB or even 1200 kB.

Those e-mails have a header entry like this:

Received: from mail by [[MY_SERVER]] with spam-scanned (Exim 4.80 #2 
(Debian))
    id 1dknc6-0006nP-BB
    for <[[MY_ADDRESS]]>; Thu, 24 Aug 2017 10:35:42 +0200

but they are not scanned and there are no entries made by SpamAssassin.

I tried to find solutions in a Debian forum, but nobody could help me.

- Why are those e-mails skipped?

- How comes "spam-scanned" into the header if the e-mail was not
scanned?

- Is there a size limit which I can change?

- If there is no solution - how can I include a small self-written Perl
script as additional filter?

Thanks in advance
Paul Lenz