[exim] Problems with Error 421

Ever since I upgraded to Exim 4.66 and my server load has gone up I've been
experiencing a lot of Error 421's (disconnection errors).

>From my google search it seems we may be memory/cpu/disk bound, and there
are suggestions on limiting the file size scanned, removing large rulesets,
etc..

I have searched the exim site looking for a way to limit the file size but I
can't find anything. Can someone suggest what I need to do to my
spamassassin transport config to limit the size of messages that are sent to
spam assassin to 100k (or less) and send the remaining though un-scanned.

Here's the exerpt from my exim.conf:

spamcheck:
driver = pipe
batch_max = 100
command = /usr/sbin/exim -oMr spam-scanned -bS
current_directory = "/tmp"
group = mail
home_directory = "/tmp"
log_output
message_prefix =
message_suffix =
return_fail_output
no_return_path_add
transport_filter = /usr/bin/spamc -u
${lookup{$domain}lsearch*{/etc/virtual/domainowners}{$value}}
use_bsmtp
user = mail

Thanks!