[Exim] Message Size Limits and bounces

We have had an influx of people who are sending in very large email
messages, 20-35mb to a number of our users. Exim fronts our mail system
on the outside and is used internally for the mail hub as well. All
regular user mail is in exchange however and there, there is a limit of
about 7mb including headers for any inbound email to a user.

So.. the mail arrives at the gateway, gets passed on to the and gets
denied/bounced at the Exchange server.

The bounce is as big as the mail arriving was despite the fact that the
bounce_return_size_limit default is supposed to be 100k.

My question is, is the reason why the return is so large, because Exim
didnt actually generate the bounce, the Exchange server did and it simply
is transporting the error (sometimes 20+mb's worth) back to the sender?

If that is the case, it would explain why when I change the
bounce_return_size_limit it appears to have no effect on these types of
inbound files hitting the Exchange servers.