Megabyte attachments are an increasing feature of email, and it is easy to leave them attached, which means that email archives fill up rapidly. When one notices and tries to address the problem by deleting old attachments (but not the associated email), it is clear that powerful tools for doing this are not really there. The add-ons which deal with this area appear not to work in the most recent releases of Thunderbird.
It might be useful if, when a message with a large attachment is being saved (to sent mail or some other folder), there were an option not to include attachments if larger than a specified amount. (Large files being attached to an email one is sending are almost certainly held elsewhere in one's file store.)