We are on Exchange 2010 SP1 and the users experiencing this issue are running Outlook 2007 SP2.
They create a recurring monthly meeting and successfully make changes to a few of the occurrences, such as adding an attachment or changing the time or location. But at some point they try to make another change to an occurrence and receive the error 'The attachment size exceeds the allowable limit'.
I found that the meeting itself is being inserted into the meeting several times, as an attachment, causing the size of the update to be too large to email (our limit is 20 MB). They can delete the attachments, but it takes a long time in Outlook. Deleting them in OWA is much quicker.
My question is why is the meeting automatically inserting itself into the meeting? Our heavy calendar users are getting to the point they won't use recurring meetings due to this issue.